It is (or was?) possible to change the design of the Editor to some extent via your own custom USS files (as explained here by a former staff). This only worked on/for IMGUI elements, however.
Even if the Editor was fully based on UI Toolkit, I doubt that we would ever see official support for such kind of customisation. IIRC, I was inquiring to… I think SimonDufour or uDamian… years ago about if it was possible to edit the Editor’s own UI Toolkit-based elements via custom USS and bottom line was: They’re hard coded and also it’s not something Unity wants to see us doing.