This question looks like a deja vu (http://www.question2answer.org/qa/43212) :)
Anyway, you can't change that color directly from the CSS because it is part of the image itself. What you could do is modify the image applying whatever background you want or maybe, even better, modify the image and make the background transparent and then modify the qa-search-button CSS class like this:
.qa-search-button {
background-color: red;
}
Then you could just change the background from the CSS itself.