How do I change the terrible background graphic behind the toolbars?

Having a dark, visually annoying and distracting background graphic at the top of the browser window is difficult for me to understand. I have used Firefox since the beginning, but have used it only occasionally since the dark background graphic made its appearance. The browser functionality is great, but I need a plain, neutral or simple background graphic at the top, to not visually compete with what I am looking at in the browser. ''How can I change this? ''