Customize eBook UI

EPUB, MOBI, FB2, and similar formats use SumatraPDF’s eBook UI (HTML-based layout). Since version 3.4 the engine is MuPDF-based: text selection, in-document search, and bookmarks work much like PDF.

What you can customize #

Open Settings → Advanced Options… and edit the EBookUI section:

EBookUI [
    FontSize = 0
    LayoutDx = 0
    LayoutDy = 0
    IgnoreDocumentCSS = false
    CustomCSS =
    WindowBgCol =
]

Setting	Meaning
`FontSize`	Base font size (default `8.0`; `0` = built-in default)
`LayoutDx` / `LayoutDy`	Virtual page width / height for reflow (defaults `420` / `595`)
`IgnoreDocumentCSS`	Ignore stylesheet from the EPUB (`true` = your `CustomCSS` wins)
`CustomCSS`	Extra CSS rules — often paired with `IgnoreDocumentCSS = true`
`WindowBgCol`	Canvas background around the reflowed text (ver 3.7+)

Full field reference: Advanced options / settings.

Themes and invert colors #

UI themes (Theme = ... in advanced settings) change window chrome. To change how the document looks:

Shift + I (CmdInvertColors) swaps text and background colors for the rendered page. This affects images as well as text — there is no built-in “invert text only” mode.
FixedPageUI.InvertColors = true persists invert for PDF-style rendering; ebook pages respect the same invert when shown through shared color paths.

For dark reading without crushing photos, experiment with CustomCSS (dark background, light text) and IgnoreDocumentCSS = true instead of global invert. See Customize theme colors.

Continuous vs paged view #

Use the same view commands as PDF: c toggles continuous mode; Ctrl + 6 / 7 / 8 change column layout. See Scrolling and zooming.

In-document find: Ctrl + F, F3 — see Finding text
Table of contents: F12 or command palette * prefix
Internal links: click to follow; Alt + Left to go back

Known limitations #

Report issues in discussions or the issue tracker.

Vertical writing (some Asian EPUBs): may not detect top-to-bottom / right-to-left layout correctly.
CSS features: not every EPUB3 layout feature is supported; complex fixed-layout EPUBs may look wrong.
ReloadModifiedDocuments: auto-reload when the file changes on disk does not apply to eBook UI documents.
Thumbnails in Explorer for .epub: limited compared to PDF — see installer -with-preview in Installation.

CHM files #

CHM uses a separate ChmUI section. Set ChmUI.UseFixedPageUI = true to render CHM with the PDF-style fixed-page engine instead of the HTML ebook engine.

What you can customize #

Themes and invert colors #

Continuous vs paged view #

Search and navigation #

Known limitations #

CHM files #

See also #