Actually, I have already been using it as an eBook reader. The background color is adjusted in the same way as the Kindle app, and the reading experience is very good.
Hyle I find SumatraPDF to be sort of the swiss-army knife for digital publications. And yes, we do have apps like Thorium, STDUViewer, Alfa Reader, Icecream Reader, which present much more comfortable ebook experience, however none of them has the 2 golden features of Koreader. One is to be able to create a "fonts" folder into the root directory and be able to use any fonts placed inside that folder in any given epub/mobi/text, overriding preconfigured fonts. The second killer feature is to have a "dict" folder in the root, where one places openly available dictionaries and uses them in epub/mobi/text to check on words. With those 2 features and with some minor settings like font size, background color, SumatraPDF will become the only epub reader to rival Koreader on the Windows platform, especially since there are NO INTENTIONS for the foreseeable future to port Koreader to Windows.
Very good suggestion and if I may add to it, the option to pick a Font for display. That will make Sumatra a competent ebook reader.