Add syntax background color variable


#1

I want to create a ui theme where the tabs have the background color of the user’s syntax theme like Sublime Text default theme does.

The problem is that there is no variable for that, every syntax themes just set a background color without a variable.

The solution could be to add a variable like @syntax-background-color in ui-variables.less with a default value.
The syntax themes will have override the value of this variable with the background color they want.
Then we could use the syntax themes background color in ui themes.