Very easily, change this line:
	PPGeDraw4Patch(I_BUTTON, 0, 0, 480, 272, CalcFadedColor(0xcFFFFFFF));
in PSPDialog.cpp to something like:
	PPGeDrawRect(0, 0, 480, 272, CalcFadedColor(0x90606060));

In fact, I just did :) Feel free to tweak the color until you think it's PSP-like enough.