SumatraPDF eBook Night Mode Not Working: How to Fix It

If SumatraPDF opens an EPUB or MOBI file with a bright page even after you select a dark application theme, the program may not be malfunctioning. SumatraPDF distinguishes between interface colors, eBook content colors, and fixed-page document rendering. Publisher-defined EPUB styles can also override the colors you expect. The practical fix is to identify which layer controls the bright area, test a simple eBook, and then adjust or safely reset the relevant settings.

1. Confirm the Symptom With a Simple eBook Test

Begin by identifying exactly what remains bright. This avoids applying a PDF-specific fix to an eBook rendering problem.

1.1 Check Whether the Interface or the Reading Page Is Bright

Open the affected eBook and compare the page with SumatraPDF's tabs, toolbar, menus, and window frame. These areas do not necessarily use the same color setting.

• If the toolbar and tabs are dark but the reading page is bright, the application theme is working. The problem concerns eBook content colors or publisher styling.
• If both the interface and the page are bright, the selected theme may not have been applied, saved, or loaded by the copy of SumatraPDF you opened.
• If only one book has unexpected colors, its embedded EPUB styles are the leading suspect.
• If every EPUB and MOBI file remains bright, inspect SumatraPDF's eBook UI configuration and settings persistence.

Success at this stage means you can describe the issue as either an interface-theme problem, a general eBook color problem, or a document-specific problem. Once you know which category applies, stop changing unrelated settings.

1.2 Test a Simple EPUB

Open a basic, DRM-free EPUB that contains ordinary text and minimal formatting. Use a file you created yourself, obtained from a legitimate source, or already have permission to read. A simple EPUB is useful because elaborate commercial books often contain embedded style sheets, background colors, images, or specially designed page elements.

If the simple EPUB uses your chosen dark background and light text, SumatraPDF's general eBook configuration is working. The original book's CSS or content design is probably overriding part of the presentation. If the simple EPUB is also bright, continue with the settings checks below.

2. Configure the eBook UI Instead of Only Changing the Theme

One of the most common causes of SumatraPDF eBook night mode not working is the assumption that a dark application theme must also recolor EPUB content. A theme generally controls program interface elements. Reflowable eBook content is handled through the eBook UI configuration and the book's own styles.

2.1 Open the Advanced Settings

In SumatraPDF, open the Settings menu and choose Advanced Options. This opens the active SumatraPDF-settings.txt file in a text editor. Close SumatraPDF before saving manual changes so the program does not overwrite them when it exits.

Look for an eBook-related section, commonly named EbookUI. Depending on the SumatraPDF build and configuration, it can contain entries for the font, text color, background color, font size, and fixed-page behavior. Do not add options copied from an unrelated or much older configuration unless they are supported by the settings file generated by your installation.

2.2 Choose Readable Background and Text Colors

For a night-style reading view, use a dark background and a light foreground color. Avoid pure black behind pure white if the contrast feels harsh. A dark gray background with warm white or light gray text is often easier to read.

For example, a configuration may use a background similar to #202124 and text similar to #E8EAED. These are examples, not mandatory values. Preserve the exact setting names and syntax already present in your generated file, changing only the color values.

After saving, reopen SumatraPDF and load the simple EPUB again. Success means the main reading area now uses the selected dark background and light text. If that happens across ordinary EPUB files, no further reset or reinstall is necessary.

2.3 Understand Fixed-Page and Reflowable Reading

EPUB and MOBI books are commonly reflowable, meaning SumatraPDF lays out their text according to the reading window, font, and size. PDFs are usually fixed-page documents whose text, images, and backgrounds are positioned on a defined page.

This distinction matters because PDF color inversion, fixed-page background settings, or commands intended for PDF rendering may not control reflowable eBook colors. If a suggested fix mentions inverting PDF pages, changing the PDF canvas, or altering fixed-page rendering, do not expect it to solve an EPUB reading-view problem.

3. Check the Source Document, Format, and Program Copy

If the eBook configuration works with a test file but not with the original book, investigate the document and the exact SumatraPDF executable that Windows is launching.

3.1 Publisher CSS Can Override Your Preferred Colors

An EPUB is a package containing HTML-like content, style sheets, images, fonts, and metadata. Publishers can assign text colors and backgrounds to the body, chapters, boxes, tables, headings, or individual spans. Some books also use background images that resemble a colored page.

As a result, a global dark background can coexist with black text, a white content panel, or other low-contrast elements. This is especially likely when only certain chapters, callout boxes, footnotes, or tables look wrong.

Compare the affected book with the simple EPUB:

• If the test book is dark and the affected book is bright, the eBook's styling is controlling the result.
• If most of the affected book is dark but isolated boxes remain white, those elements probably have explicit styles.
• If a page is actually an image, changing text and background colors will not recolor pixels inside that image.

At this point, SumatraPDF may be behaving as designed. Try another legitimate reader if you need a reader that applies stronger author-style overrides. Do not remove DRM or bypass document restrictions to edit the file.

3.2 Confirm the File Is Really an eBook

A file's visible name does not always tell you how it is rendered. Confirm the extension and format shown by Windows and SumatraPDF. A PDF packaged or downloaded alongside eBooks is still a fixed-page PDF. A comic archive is image-based, while a CHM file uses a different content system. MOBI support and presentation can also differ from EPUB because the formats are not identical.

If the document is actually a PDF, use the settings appropriate to fixed pages. If it is an EPUB or MOBI, continue using eBook UI troubleshooting. Success means the setting being changed corresponds to the document type being displayed.

3.3 Check Windows File Associations

A Windows file association can launch a different SumatraPDF executable from the one you configured. This happens when an older installed copy and a newer portable copy both exist, or when shortcuts point to different folders.

Open the eBook through the SumatraPDF window you just configured rather than double-clicking it in File Explorer. If night colors work when opened from that window but not when the file is double-clicked, Windows is probably launching another copy.

Check the target of your SumatraPDF shortcut and the default application assigned to EPUB or MOBI files. Associate the format with the executable whose settings you intend to use. Once both opening methods produce the same colors, stop troubleshooting the document itself.

3.4 Know Which External Factors Are Usually Irrelevant

Ghostscript, printer drivers, and video codecs are not normal dependencies for displaying reflowable EPUB text colors in SumatraPDF. They may matter in other document, media, conversion, or printing workflows, but installing or changing them is unlikely to fix a bright eBook reading page.

Windows contrast themes, display color filters, and graphics settings can affect the appearance of the entire screen. However, if only the eBook page is bright while other applications and SumatraPDF's interface look correct, focus on eBook settings and publisher CSS first.

4. Inspect or Reset SumatraPDF-settings.txt Safely

If changes disappear after restarting SumatraPDF, the issue is likely settings persistence rather than eBook rendering. The active settings file must be writable, and you must edit the file used by the executable that is actually running.

4.1 Find the Active Settings File

The safest way to locate the active file is to use Settings and then Advanced Options from inside the exact SumatraPDF window exhibiting the problem. An installed copy commonly keeps user settings under the current Windows user's local application data, while a portable copy may keep settings with its executable or in its portable working location.

Do not assume two copies share one configuration. Also check whether cloud synchronization, security software, folder permissions, or a read-only attribute prevents the file from being updated.

4.2 Back Up Before Editing

1. Open Advanced Options and note the file location.
2. Close SumatraPDF completely.
3. Copy SumatraPDF-settings.txt to a safe backup name.
4. Edit only the relevant eBook color values.
5. Save the file and reopen SumatraPDF.
6. Test the simple EPUB before opening several other documents.

Use a plain-text editor and preserve the file's structure. A missing bracket, altered section name, or malformed color value may prevent the intended setting from loading.

4.3 Reset Only When Necessary

If the file appears damaged or contains years of conflicting changes, close SumatraPDF and rename the settings file rather than deleting it. For example, rename it to SumatraPDF-settings-backup.txt. Start SumatraPDF to let it create a clean configuration, then change the eBook UI colors again.

Success means the fresh configuration retains your colors after SumatraPDF is closed and reopened. If it does, copy back only settings you genuinely need. Do not restore the entire old file immediately, because that can restore the original problem.

5. Run a Clean Temporary Test Before Reinstalling

Reinstalling is rarely the best first response to a color-setting problem because an installer may preserve the existing user configuration. A clean temporary test isolates the executable, settings, file association, and source document more effectively.

5.1 Use a Separate Portable Test

1. Download SumatraPDF only from its official website.
2. Place a portable copy in a new writable folder.
3. Do not copy your existing settings into that folder.
4. Launch the portable executable directly.
5. Configure the eBook background and text colors through its Advanced Options.
6. Open the simple EPUB from inside that window.

If the clean portable copy displays the expected night colors, your regular installation is probably using an old setting, another settings location, or a different executable through Windows file associations. Reinstallation is unnecessary until you identify which copy should remain.

If the clean copy displays the simple EPUB correctly but not the original book, the source document's CSS, images, or format is the likely limitation. If even the simple EPUB ignores valid eBook color settings in a clean copy, verify the configuration syntax against the official settings documentation and report a reproducible issue if appropriate.

6. Quick Fix Checklist

• Confirm the bright area is the eBook page, not merely the application interface.
• Test a simple EPUB with minimal publisher styling.
• Use the eBook UI color settings, not only the application theme.
• Select a dark background and a clearly contrasting light text color.
• Close SumatraPDF before manually saving SumatraPDF-settings.txt.
• Reopen the program and verify that the colors persist.
• Open the file from the configured SumatraPDF window to bypass association confusion.
• Check whether installed and portable copies use different settings.
• Suspect publisher CSS when only one book or certain elements remain bright.
• Do not use PDF inversion instructions as an EPUB night-mode fix.
• Rename and regenerate the settings file only after making a backup.
• Run a clean portable test before reinstalling the application.

7. Frequently Asked Questions

7.1 Why Is My SumatraPDF Theme Dark but My EPUB Still White?

The application theme and eBook content colors are separate presentation layers. A dark theme can change menus, tabs, or other interface elements without changing the reflowable reading page. Configure the eBook UI background and text colors, then test a simple EPUB.

7.2 Why Does Night Mode Work With One EPUB but Not Another?

The affected EPUB may contain publisher CSS that explicitly assigns a white background, dark text, colored panels, or background images. If a simple EPUB follows your chosen colors, the global configuration is working. Stop resetting SumatraPDF and treat the remaining behavior as document-specific.

7.3 Can I Use a PDF Dark-Mode Fix for EPUB Files?

Usually not. PDF pages use a fixed layout, while EPUB content is commonly reflowable. PDF inversion commands and fixed-page canvas settings do not necessarily control the eBook UI. Always confirm the actual document format before following a dark-mode tutorial.

7.4 Why Do My eBook Color Settings Disappear After Restarting?

You may be editing the wrong settings file, saving while SumatraPDF is still open, or writing to a protected or read-only location. Open Advanced Options from the affected program window, close SumatraPDF, edit and save that file, then reopen the application.

7.5 Should I Reinstall SumatraPDF?

Not initially. Reinstallation may retain the same settings and file associations. First test a clean portable copy in a separate writable folder. Reinstall only if a clean test indicates that the installed program files are the problem rather than the configuration or eBook.

7.6 What Does a Successful Fix Look Like?

A successful fix makes a simple EPUB open with your selected dark background and readable light text, and those choices remain after restarting SumatraPDF. If only one publication still looks wrong, the problem is probably embedded styling. At that point, stop changing global settings because they are already working.