Reposted with permission: a student here at Cal Poly wasn't overwhelmingly happy with the default color schemes, came up with this one instead:
Here's the source:
edit->preferences->color->
Background tab
Background = #1F2C33
Racket tab
Symbol = #F48CB9
Keyword = #ABABF9, Bold
Comment = #F4C994
String = #7AD37A, Italic
Text = #33AD33
Constant = #8BD38B
Keyword = #F7F093
Parenthesis = #4FD5AE
Error = #FE0000
Other = #FEFEFE
HtDP Languages tab
Tests Covered = #B8FD9E
Tests Didn't Cover =
Foreground Color = #FE0000
Background Color = #000000
REPL tab
Values = #7A51E1
Errors = #FE6565
Output = #F28AB7
Nice!