Google doesn't consider you to be a programmer unless you append one of the frameworks, languages or MSDN to the search query. I search for "
create themepack file msdn[
^]" and it yielded that there is no C# library (or even C++ one). There is just a format that you have to follow. You can use C# to generate the file;
plain-string-text. Add the images and other settings to it, and you're good to go.
Have a look at this file format description on MSDN,
Theme File Format (Windows)[
^]
Read this SO thread,
How do I change the current Windows theme programatically? - Stack Overflow[
^]