Sumatra has 2 UIs:
  • one for PDF/DjVu/Comic Book documents
  • another one for eBook documents (ePub, mobi etc.)
As of version 3.1.2 eBook UI doesn't have zoom (changing font size), text selection etc.
There are 2 ways to customize eBook UI.

Use advanced settings

Use Settings -> Advanced Settings menu and find EbookUI section.
As described in http://sumatrapdfreader.org/settings.html, you can change the following aspects of eBook UI:
  • FontName
  • FontSize
  • TextColor
  • BackgroundColor

Use PDF UI for eBooks

Use Settings -> Advanced Settings menu and find EbookUI section.
Set UseFixedPageUI = true
Restart the app after changing the settings.

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