Styles editor export CSS settings

[elig0n]

Description

I often find my self wanting to save my latest CSS modifications to a file, i.e. to restore certain settings after further modifications.
I suggest adding such export button near the existing "Reset CSS variables" button that will either offer to save to file or copy to clipboard.
This way all users can backup & restore their style customizations, support theming and such.
Hopefully in the future it would be implemented to also read & load them back to the interface but for now I guess they could be put in the Custom CSS text box.

[emvaized]

You can use the existing "Export addon data" button at the bottom of Sidebery settings