Global Styles: Clarify what content and wide settings do

[creativecoder]

What problem does this address?

From observing theme builders recently, the Global Styles Layout settings are usually confusing without prior knowledge. One initially thought "Content" and "Wide" might refer to responsive styles.

Even if you start changing the settings to observe the effect, you may or may not see any change, depending upon what blocks are visible on the canvas at that time.

What is your proposed solution?

I'm not sure!

One idea: show a semi-transparent overlay on the page (like a vertical bar on each side of the page) when adjusting Content and Wide, so you can see that a width is changing, independent of what blocks are visible.

Related to #52790