It is not advised to change the installed CSS. The way you should change this is to create your own CSS that overwrites the default.
In APEX there are various places you add custom CSS. In every page definition you have a section called css where you can include either file urls or inline css. This blog post I wrote a little while ago may also help, you can link css urls in the page template so the css will be added to every page using that template:
You can either use a file stored on your own server or one uploaded in shared components under css. To reference a file in shared components use the substitution string:
<link rel="stylesheet" type="text/css" href="#WORKSPACE_IMAGES#MyUpload.css">
Hope this helps.
these CSS are found in themes that ship with the version of Apex that is installed in the folder IMAGES. However much advise it said not to alter these themes but rather create custom CSS to override some defaults in the theme you are using. A good way to do this to use developer toolbar from Chrome or firebug in Mozilla Firefox, to inspect various elements of the page and where there is a need to modify, then u can create custom CSS for this Following what Paul described above.