I found I really did not like SD Dark. Waay too contrasty and eye-strainy.
So I switched to Discourse Classic.
It looks very little like vBulletin used to. But now 5 years after the switch to Discourse, I really don’t care what vBulletin looked like. When I visit other vBulletin-based boards I find the look & feel positively antiquated.
My expectation is that Discourse Classic is used throughout Discourse, not just here in our trivial corner of their vast universe. As such it’ll be a) stable, b) adequately QC-ed & tested before a change is released, and c) quickly fixed if broken.
My advice: If anyone here is struggling with finding a no-hassle theme, try that one.