If I recall correctly, since SuiteCRM 8 there is only one theme available by default (others have been dropped).
I’ve seen some posts on the forums here, where someone used something like this:
to add some small changes to the look and feel of the CRM.
Overall, you can develop your own theme - might be a bit risky / a lot of work for little business value, but it will work with enough effort.
Check out this one for inspiration:
However, this theme on a VueJS basis is build for Suite7