I think there are different themes at a slight level? When setting up my instance I left it at “browser default”, but I’ve been to instances that had something else chosen, apparently. When I see it in the browser it’s a pretty efficient dark theme. These are the options in the instance setup, maybe your instance had one of these enabled:

if you willing to install greasemonkey / tampermonkey to resolve it, you might be interested in: https://sh.itjust.works/post/70273

I am using Mlem on iOS - everything else is hardly working for me. Mlem has Barebone Apollo Vibes tho.