What's wrong with creating a custom theme the way that they provide?
Settings -> Themes -> Choose your own colours
If they had done it using the 'style' attribute, which they haven't (but other sites may have), then using this
jQuery Selector[
^] may be of interest.
Actually, I had a brain wave.
Using jQuery, select everything:
$('*').each(function(){
if ($(this).css('background-color')=='blue')
$(this).css('background-color','olive-green');
});
Note that I think the css function returns colors in RGB... but don't hold me to it! I also haven't tested it, but I don't see why it shouldn't work (famous last words!).