Hello!  I am new to designing for Salesforce.  We are trying to change the look of our Partner Portal (PRM).  From looking at the source code & CSS files, I can tell that something in the common.css file is affecting the PRM layout (specifically the width of a column -- we'd like to edit this value). 

Is it possible to edit this file?

If so, where can I find it?

Is there a better way to "override" the class of that stylesheet?

Thank you!