Docs: Page background color setting is hard to discover

[king-of-the-grackles]

Problem

When trying to change the Auth Hosting page background color, users naturally look in Styles/Themes settings first. The theme JSON has globals.colors.background which appears to be the right place but doesn't control the page background.

The actual solution (the bg URL parameter) is only documented in Auth Hosting docs and isn't cross-referenced in Styles/Themes documentation.

Suggestion

Add a note in the Styles & Themes documentation that for Auth Hosting, the page background is controlled via the bg URL parameter, with a link to the Auth Hosting docs.

This would save users significant time debugging why their theme changes don't affect the page background.