We're a Lemmy instance, that's part of the Lemmy design. I'd suggest posting on the main instance under their support if you'd like to give them feedback to improve the user interface

I'm using dark mode, and it looks fine. But it does look like they use the same text colour for light and dark modes. I'd guess the Lemmy developers use dark mode and no one has complained yet (or if they have, they haven't got around to changing it).

If you weren't aware, you can click your name at the top right, choose Settings, then change the theme on that page. There are a few different ones to choose from.

dark mode ftw