I'll second Ubooquity. I have a lot of experience with this, as I've been self-hosting my eBooks and Audiobooks for many years now. Ubooquity is not perfect, but if you're willing to tinker with it, you can get it set up pretty nicely. There are themes, and the Plex theme actually makes it look really slick.
Kavita is the new kid on the block for me - I have been testing it out as a general-purpose eReader but I'm not ready to give it my recommendation yet.