Ok, Heres something interesting. I ran it in all styles including plastique to see what would happen, and I got the exact same results as I described with the windows XP style, except in the CDE style. Heres what happened:

-When the form is untouched in CDE and classic windows style, the comboboxes are raised, but they are white. When the test slot is activated, the ui.vendor_state combobox becomes grey, and the ui.vendor_zip line edit becomes yellow.

-In the classic windows style, the same thing happens as in the CDE, except instead of the whole combobox turning grey on activation of the slot, only the tiny drop arrow on the right hand side turns from white to grey.

I notice that the plastique or cleanlooks style, the comboboxes are a shiny gray color when you are not in the process of selecting an item from them. It seems like this can't be changed.

In the windows XP style, the comboboxes are text with a white background but still won't take the style. They look like they could be changed, but maybe they somehow inherit the same traits as plastique/cleanlooks.