Simple browser-side CSS does the trick. I use Stylish[1] for Chromium. I believe there is also a Firefox version.

[1] - https://chrome.google.com/webstore/detail/stylish/fjnbnpbmke...

I changed django admin's base.css:

    #header {
       ... css ...
       background: <whatever color you want>;
       ... more css ...
    }