More dark mode issues

[sorawee]

• Check Syntax Error Message still has white background:

• Package manager's terminal still has white background (perhaps the fix is to change https://docs.racket-lang.org/mrlib/Terminal_Window.html?)

[rfindler]

That "other mode" of check syntax (where it doesn't run in a separate place) just needs to be ripped out of DrRacket.

And yes, I think that the terminal window needs to support dark mode. I see that it already depends on the framework so there won't be any new dependencies, happily.