Making a .Html file compatible with a .Css file

Hey. After shortening everything that I found possible I am back and have found that my header.html file is incompatible with the stylesheet 'common-style'. I need these two files to be compatible to be able to merge the header with my other files. If you find these files to be difficult to edit because of the sheer length I would appreciate some help on how to make the files even more shorter (While maintaining compatibility). Thanks for the help.

The 'Header' Html File:

Expertpcguides.com

The 'Header' Styling:

http://www.expertpcguides.com/css/header-styling.css

The .CSS Stylesheet Which the header is incompatible with:

http://www.expertpcguides.com/css/common-style.css