There is no need to replace or edit the existing CSS Styles. Addition of a Local CSS File will render your desired effect.
CSS works on the principle of overriding styles. What happens is that all the linked css files get loaded into the browser, and after all the standard files, the "css tab" (or local css as we call it) gets loaded. Local css does not "replace" any of the css files, it simply gets loaded after. Any styles you define in it will override our standard css if they are the same styles. You do not need to paste the entire content of a css file if you only want to change 2 rules in the file, just paste the relevant rules with the changes you want, you don’t even need to include attributes you are not changing.
This will override only the background color, and leave all our other styling as they were. Minimizing what you put in that file is actually the preferred way of doing things, since this way there is a lower risk of your website breaking should we update our template stylesheets in order to fix a bug that’s affecting all of our clients.
Please see How can I add my Local CSS File? for further instructions.