I recently altered my Firefox preferences to display a custom peach color background. Initially the purpose was to ensure that my designs were displaying their CSS background colors properly.
Since that time, I have noticed other web pages were displaying my custom background instead of what the designers thought they had designed.
All it takes is a simple background declaration in the CSS file and this problem can be fixed.
body {background: #fff;}
So many designers either forget or take for granted that their default browser background color (usually white) will be the same default color for all their visitors. No so!
To change the settings in Firefox to display a custom background

I couldn’t find a preference setting for Safari, but for Opera users, choose Preferences>Web Pages.
You will be amazed at how many web sites will now display your custom background color. I have started a list of high profile pages: