So in part one of this post we got all the CSS and other theme file we needed to stuck them in a document library.
This line of code basically finds all the linked CSS files in the page and changes thier value to point to the new style sheet you select. This is fine on a site that just uses one CSS file but in SharePoint bad things happen. In most cases we dont want to completely remove the reference to core.css. For most themes many elements don’t need to be changed so we only have to write the CSS to change them and let core.css do the rest. If you remove core.css its like using a global reset (discussed in more detail here) which means you will have to restyle everything. So them point is we want to leave core.css in the page and just add our theme.
To do this I had to make some changes to the master page to create to interface users will use to swap themes.