WBA themes with the file extension *.wba are WindowBlinds themes. *.wba files are basically zip files that can be used by the program WindowBlinds.
WindowBlinds is a great tool with high-quality shell themes, we reported about it on a few occasions here. WBA themes are often high-quality themes and the software to use them will set you back a few bucks.
To install the themes, simply buy WindowBlinds and all *.wba files will be automatically applied when you double-click them.