I have Syncthing installed on several servers (all windows). I don’t like the default white theme, so I switch to the Dark-theme. On most servers this works, but on some, whatever I set the theme to, nothing changes and the page stays with white background. Going back to settings or even checking the config.xml confirms the setting does get set, so it’s not the setting itself causing it: 127.0.0.1:8384 dark
I think this is fixed in the next release, but you can just Shift+F5 to force refresh.