Apply it where?

While in development - add it to the module?

Or in implementation - apply it to a module?

Unfortunatly if the module itself does not support it you will not be able to do it.

What you need are the appropriate css classes to hookup to. But these are added by developers at development time so that they can be changed later through different style sheets.

Try to find the styles of the module you will be using and then you can apply your changes on a few different levels - check the skinning documenation for a great explanation of css styling in dotnetnuke, and how do the styles work exactly.