Theme: Set color on wrapper

[mirka]

Part of #44116

What?

Set the themed foreground color as the color for the wrapper div on the Theme component.

Why?

While working on #58014, I noticed that the Theme component does not set the foreground color as a color for its wrapper div.

How?

Sets currentColor as the fallback when --wp-components-color-foreground is not defined. This can happen when the consumer has not provided a background prop on Theme.

I briefly considered falling back to GRAY[ 900 ], but that would probably be a bit too intrusive 🤔 We can revisit this if necessary.

Testing Instructions

1. Look at a Storybook story with some text that is not explicitly colored, e.g. the View component.

2. Using the Theme switcher in the toolbar, try out the different presets.

   • For the two presets that set a background color, the text color should responsively change to either #ffffff or #1e1e1e depending on the background.
   • In the "Classic" preset that only sets an accent color, the text color will stay as #000000 (= currentColor).
   • In trunk, the text colors won't change at all.

CleanShot.2024-01-23.at.06.45.17.mp4

[ciampo]

I briefly considered falling back to GRAY[ 900 ], but that would probably be a bit too intrusive 🤔 We can revisit this if necessary.

Agreed, this PR represents an improvement over what's on trunk anyway,