Exporting UI theme to .jar file

I want to change a single element of the UI theme I am using. I found the source code for the theme online, as the creator has kindly uploaded it to GitHub.

I made the changes that I wanted and now I have a folder of files, which I tried to export to both a zip and a jar file manually. However, when I try to import that file, I get the error 'failed to load plugin descriptor'.

I have looked at the docs and I found nothing pertinent to what I want to do -- only a tutorial on creating new UI themes.

What should I do?