Having difficulty with a simple issue

I wish to have white text for most of my forums, which mean the background for typing posts needs to be a different color. I've entered Display >> Pictures and Colors and selected a color I like for the form fields, and it works perfectly as I envision things in the PREVIEW mode. However, the actual forums remain unchanged.

Here is an example; https://2img.net/r/ihimizer/img202/9282/issueu.jpg
The top one is what I would hope for it to look like, the bottom is how it looks outside of the preview mode.

How would I go about fixing such an issue? And thanks in advance!