also:
* simplified CSS selectors where possible
* .config-name = var name, 1st element in <label>
* .config-value = var value, 2nd element in <label>
* .nondefault class added on <label>
* same icon in manager
(didn't change the editor config icons as those aren't for usercss)
* "external" icon for USO styles to avoid confusion
(opening a tab is an extremely different action)
* smaller paddings in popup
* removed hover effect on label text
(necause native <select> elements cannot be expanded from js)
* various fixups
* don't close on clicking "save" to allow continuous tweaking
* dirty item's label is marked with * and switches to italic
* "save" button is enabled when some value differs from saved
* "use default" button is enabled when some value differs from its default
* "close" becomes "cancel" when there are unsaved changed values
