Select (dropdown) CSS

I’m struggling to identify what CSS to use to change the background and font colours of a Select dropdown on my WC Chechout page. (“How did you find us” on this page.)
At the moment the dropdown options are white text on a white background until the cursor rolls over them. They don’t seem to be effected by the same CSS as all the other fields on the page.

Can anyone help please?

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