I wouldn't just use a CSS theme, but something with bolts included (navigation, state, charts, UI widgets, animations, etc.). In other words, something with styling AND interactivity code already built in.
For work, we're moving an existing homebrewed system to one of these: https://mui.com/store/#populars
It builds on top of MUI, which is a collection of ready-to-use React UI widgets that are pretty well designed (based on Material) and implemented in terms of UX/UI details. If you're not familiar with those nuances, MUI gives you a very good starting point that will meet 90% of needs out of the box, but is still very extensible if you ever do need to customize something. The one downside is that your dashboard can end up looking a bit generic, "Material" style, but that's easily fixed with some CSS tweaks here and there. The reverse, starting from a CSS-only theme and building up around it, is much harder.
Then the dashboard templates build further on top of that to give you ready to use data widgets that you can just hook up to your backend. I think using something like this can save you hundreds of hours from not having to reinvent frontend basics for a basic dashboard. Just my 2¢.