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.