+1  A: 

Simple answer, you can't.

Styling drop-down lists, scrollbars and such will almost always leave you disappointed in some/most browsers.

I would just leave it to the theme to decide.

Cyrodor
Agreed. The dropdown arrow, scrollbars, how the 'check' looks in a checkbox - these are beyond the CSS spec. Leaving them to the OS defaults, /whatever they may be/, is best.
Jon Cram