I just stumbled upon this and it's almost perfect for me. (no theme colors customization)
Thorium Reader
https://www.edrlab.org/software/thorium-reader/
Windows, MaxOS, Linux
Thorium Reader is the EPUB reader of choice for Windows 10, MacOS and Linux.
This EDRLab application is in constant development and aims at becoming a reference for accessing EPUB 3 publications in reflow or fixed layout format, audiobooks and visual narratives, LCP protected or not.
It is localized in a large set of languages, each version offering new languages.
Huge efforts are also made to get Thorium Reader highly accessible for visually impaired and dyslexic people.
This application is free, with no ads and no leaks of private data.
This is the perfect tools for heavy readers, library patrons and students.
They also got open source toolkits in case any devs are interested. (*cough* calibre-web? *cough*)