Did you try out all the themes available in your profile? Lemmy comes with a bunch by default but server owners can install custom ("extra") ones. Lemmy-ui is the place to contribute new themes you want default, but keep in mind there used to be more themes before they deleted a bunch due to maintenance costs.
If you want something totally custom for yourself then a stylish type route is probably the only (outside standing up your own instance) but that and extra themes both will need to be tweaked as new versions of lemmy come out. Lemmy 0.18 for instance involved an upgrade to a new version of bootstrap that mildly broke some theming people had. Nothing prevents lemmy from totally overhauling themes and totally breaking any work you put in.