ComponentSoftware Diff (CSDiff) is a software utility that makes comparing WordPress folders or files easy—and it’s a no-cost download, too. CSDIFF is not a WordPress plugin but is a stand-alone utility.
I have implemented WordPress versions 2.5x , 2.6x, 2.7x–and the latest version, 3.x. Finding the differences between between the older CSS and template files and those of a newer version can make an upgrade easier. CSDIFF makes finding the changes easy. As an added bonus, the difference-analysis report generated by CSDIFF can make documenting changes a snap.