It was in Fedora's repo, so I tried on a Fedora machine I have. It installed and worked. And does have a dark mode, it respected system preferences which was set to dark mode in KDE settings!

But it no option menu and no obvious way to turn pages without using the table of contents tool. It didn't respond intuitively to mouse clicks or touch gestures attempts to turn the pages. I will try it some more later, but it doesn't follow hardware ereader (tap to turn page) or normal desktop (no visible buttons or click/drag) UI conventions.