So we have a working theme for XenForo 2.3.4 now. You can test-drive it by scrolling down to the bottom of the page and choosing "FBG2025" from the "Style Chooser" (the icon looks like a paintbrush) in the bottom left.
One nice thing is the new style will automatically mirror your devices' light/dark setting, so you shouldn't have to change that setting unless you're used to viewing the boards in opposition to your default light/dark setting. If you do want to change your light/dark setting, it's just to the right of the "Style Chooser" -- the icon is supposed to be the sun. OK, "light", I get it, but it looks like the cog/gear icon everyone uses for "settings" these days. Anyway, you can click that and choose "System", "Light", or "Dark" and that setting should persist for your account wherever you log in to the boards. You can also pick your "Style Variation" from the "sun" at the top right of the page next to the search/magnifying glass icon.