How to set background colour for logo with transparent background

Hi,
I have a customized login page with a logo that has a tranparent background. I was wondering how I can set the background colour – for the logo only (!) – to White.

The idea is that my logo with a white backgorund connects seamlessly with the login_form that also has a white background.

Thanks,
Jasper

The page I need help with: [log in to see the link]