Not sure when they added it, but you can get dark mode on it. The path for me: C:\Program Files (x86)\Microsoft SQL Server Management Studio 19\Common7\IDE the file you need to modify is ssms.pkgundef and the line has a comment above it that says //Remove Dark Theme
Just comment the line. Boom
Edit: I have a very new edition of ssms (some version of 19) so YMMV