You just need to specify a background colour for the input fields in you css. I've just spent some time getting rid of the colours in the form fields in the Contact Form Solution Pack contact_styled.php file. They were especially annoying! (Here)
EDIT: Actually, no. The form fields in the WebAssist log-in page are secified by css, so something about your system must be 'over-riding' it. What happens if you preview in other browsers?
EDIT 2: You could put your mind at ease by giving your form-fields background-images (just a tiny coloured (or white) image set to repeat x + y). I'd like to see the browser get past that!