[feature] Add support to configure background color of the status bar item for different modes

[gayanper]

Is your feature request related to a problem? Please describe.
The status bar item for mode is not prominent among other items in the status bar

Describe the solution you'd like
Introduce new themable properties for status bar item for mode so that one could either configure it in settings or depend on a theme that supports it.

Describe alternatives you've considered
Right now I use to patch the main vscode code workbench css file to configure colors by looking at the aria label of the item.