Alabaster Variant theme

This theme is inspired by the motivation of Tonsky's Alabaster. In fact, most of the light variant of this theme is lifted from his own extension. I decided to make this theme for two main reasons:

1. I was irritated that there wasn't a single extension providing both a light and dark variant that were consistent.

2. I didn't like the UI color scheme for existing extensions.

I'm not generally a fan of "flattening" the UI scheme; Where many UI elements have the same background color. Nor am I a fan of "tinted" themes, where the base colors are something other than shades of gray. The themes provided here have non-tinted themes, where the elements are separated rather than flattened.