Hubitat Dashboard does provide you with a way to specify custom CSS. Some people use this to tweak things they don't like or add customizations that they do like. I rarely use dashboards in the first place and am OK with the defaults for pretty much everything, but there are some threads where you can find neat things people have done with this or other customization you can do:
- [RELEASE] Smartly 2.0 featuring Smartly Inject (this is more than CSS but demonstrates what is possible)
- Show Off Your Dashboards! (thread with user dashboards, mostly Hubitat Dashboard)
- [Release] HubiGraphs 4.2 (not a dashboard per se but is one way you can get graphs/charts into a Dashboard, which a lot of Home Assistant users seem to do)
But I'm not sure what exact Home Assistant dashboard features you're looking to replicate. If you like them that much, you might have an easier time just exposing your Hubitat devices to Home Assistant via a community integration--then you can use the real thing. 
But you can do a lot with the built-in options if you know how, too.
