Allow user to export/import individual scrollbars

[WesleyBranton]

Users should be able to export and import individual scrollbars. This will allow them to share them with friends or transfer them to another browser instead of doing an entire backup.

This feature would be essential for a scrollbar marketplace (see issue #140), if that is something that is created in the future.

[e-t-l]

@WesleyBranton Similar to this, I'm trying to export the raw code for my own scrollbar from the extension to use in situations where extensions might be disabled. My understanding is that the scrollbar is controlled entirely by CSS. In Firefox's Developer Tools - Style Editor, I can see the style sheet for Custom Scrollbars contains the following code:

* {
	scrollbar-width: thin !important;
	scrollbar-color: #5454546d #38383800 !important;
}

But when I tried to save this in my userContent.css file (yes I have FF's legacy styleshseet pref enabled) as the following, it didn't work:

@-moz-document {
	scrollbar-width: thin !important;
	scrollbar-color: #5454546d #38383800 !important;
}

I also tried injecting the code directly as a UserCSS with Tampermonkey, but again it didn't work. Am I missing something? How would I inject the CSS for my custom scrollbar so that it would work even if the extension is inactive?