Klarine has the right idea.
The original idea behind allowing the board style to affect the phpBB2 acp was that control panel authors and webhosts in general would customize it to suit the theme of their own control panels. However this was never really widely used as such, and in fact caused issues in phpBB2 when a user breaks his template, he will possibly lose acp access (which from my experience was relatively common with new inexperienced users).
David is right, you can still skin it of course. Here is an example by DoubleJ