It's done by the built-in highlight_string() php function. So, in order to modify it, you would have to write a syntax highlighter and then change the function call to the one you wrote. It's doable, but requires hacking, a bit of a pita (mainly to code the new highlighter) imho.
Basicly for the curious, search for <i>phphighlite</i> in admin/functions.php, that's the part that needs to be modified.

Edit: minor hacking should let you put a white background (just change the return line).