Make the centered view margin color customizable

[isidorn]

@aeschli I want to make the centered lauout margin background color customizable. Can you please help me with the name? Here's my proposal:
centeredLayout.marginBackground

fyi @joaomoreno

[bpasero]

@isidorn I would probably keep it simply and go with something like centeredLayout.background and we only show this color in the margin area because the elements within the widget have their own background colors.

If we want to make it easier for theme authors to correlate this with the editor area, it maybe should be editorCenteredLayout.background?

Other colors to get inspiration from: https://github.com/Microsoft/vscode/blob/b7e2e55f6df49bdee4805787aa55d6b1fde08466/src/vs/workbench/common/theme.ts#L120

[aeschli]

@isidorn @bpasero What about editorPane.background?

[isidorn]

I can go with editorPane.background and we can rename next week