You can specify your own CSS file in the settings, or write the CSS code directly.
How do I make the Seafile web UI more readable for a visually impaired person? (Keyword: Dark Mode.)
There is a dark mode built into version 13. In the top-right corner of the screen there’s the user’s avatar. Left of the avatar is the notifications bell, and left of that is a sun or moon icon that switches between dark and bright mode.
You should warn the user that dark mode is new and has some problems, so you will sometimes get a white flash-bulb blast while navigating the web UI. For example going into or out of settings, or just clicking the browser’s refresh button can be blinding.