Creating aliases or mappings in CSS

I am looking for a strategy to resolve the following:

Our corporate standards folks provide ready-made CSS files that may be used for styling our web applications.

A third party application has its own idea of how it would like to name its styles and provides some CSS files to start with in order to then 'skin' their product.

If this were a one-time shot, it would be feasible to just manually create a new CSS for the third party application which uses its class names but is populated with style information copied over manually and carefully from the corporate CSS where they apply.  The risk here is that if either the corporate CSS changes or the third party default CSS changes, someone would then have to figure out what the changes were and then again manually migrate the changes over.  There has to be a better strategy than this.  The best suggestion gets the points!