It is Aug 2011, Xcode 4.1 and it continues to be nightmare. Why not allow Cmd+ and Cmd- like everywhere else. < end rant >
As suggested by other respondents I suggest that you copy your preferred theme before you tweak the fonts for there is no ease way to undo.
In my case, all I wanted to do was increase the font size. Mercifully you can select multiple items (Shift+select or Cmd+select) and set the font for all the ones selected in one fell swoop.
Minor relief for a major irritation.