Is your feature request related to a problem? Please describe.
The cumbersome installation steps are one of the reasons that prevent people from using userChrome.css.
It would be helpful to have an theme installer.

Describe the solution you'd like
I made a installer prototype, currently it only works on Linux, but it is easy to make it support other platforms.

If you are interested, I can move the repo to FirefoxCSS-Store and improve it together.

Describe alternatives you've considered
N/A

Additional context
N/A