Adding new module

[disconnect]

Hey, big time newbie here trying to customize the top menu.

To do that I need to edit superfish-modified.css (module topmenu). However the changes are irreversable, so I want to make a backup: create a backup module. AS far as I understand, I have to upload it and change the 

<link href="/modules/blocktopmenu/css/superfish-modified.css" rel="stylesheet" type="text/css" media="all" />

code to link to my new blocktopmenu_tweaked module. However I have no clue how to do that (this is a line from "view page source" on index.php. It is generated dynamically somewhere, isn't it?). Please help

Edited December 13, 2013 by disconnect (see edit history)