I’ve added a couple of screenshots in the following comments.

Just a quick update: I added an alternative ePub reader that respects the theme colors. (Note: it doesn't save reading progress yet).

Link in the first post.

Glad you like it!

Good news: I just updated the theme to include a Dark Mode. Instead of making a separate version, I added a toggle button (Sun/Moon icon) in the top bar so you can switch between Sepia and Dark mode whenever you want.