you need to edit the color declaration in the following rules
I just showed you the rules you need to edit, the colors you would like to use are up to you. If you need further help, please check the CSS spec.
You could use a server-side include for your menu structure so that you could keep it in a single separate file. For example, in PHP it could be achieved via the include function. If your server supports other server-side languages, please check how you could do it with those.