CSS is a standard used to define Web site presentation styles. There are some good tools that can parse and compare two CSS style sheets in order to find out which selectors have different styles. If you want to do that with manually method, it may take a long time and you can be so tired.
The following tools can assist you the CSS comparison job easier and faster:
diff (command line utility via the Terminal) http://gnuwin32.sourceforge.net/packages/diffutils.htm
4. Compare with Browser extension