Update pagure/static/pagure.css
    
    On older browsers, the gradient used in the masthead class (the top of the page) was not showing, and the user would instead get the body colour which is a very dark (almost black) grey.

    

    This commit adds a background colour to the masthead. Since the gradient is in an background-image, this colour will only show if the gradient fails.

    

    fixes #2864