Issue has been identified and a fix is being made. It will be rolled out with the next update of SSMS 18.
Same issue, INSTEAD of killing SSMS when the error window pops up just PRESS ENTER and it should un-freeze.
There is no plan to invest on the dark theme for SSMS.
Please, feel free to upvote item #33076054 and add your comments there, in case we end up bringing it back.