Improve organisation of site background options

[jameskoster]

Extracted from #62752 as a smaller step forwards.

In global styles background options are a bit dis-oraganised and in the case of background image, tricky to find. This can be addressed by adding a 'Background' section which collates both options.

The background color option can continue to live in the Color section as it does today.

Icon svg:

<svg width="24" height="24" fill="none" xmlns="http://www.w3.org/2000/svg"><path fill-rule="evenodd" clip-rule="evenodd" d="M11.53 4.47a.75.75 0 1 0-1.06 1.06l8 8a.75.75 0 1 0 1.06-1.06l-8-8Zm5 1a.75.75 0 1 0-1.06 1.06l2 2a.75.75 0 1 0 1.06-1.06l-2-2Zm-11.06 10a.75.75 0 0 1 1.06 0l2 2a.75.75 0 1 1-1.06 1.06l-2-2a.75.75 0 0 1 0-1.06Zm.06-5a.75.75 0 0 0-1.06 1.06l8 8a.75.75 0 1 0 1.06-1.06l-8-8Zm-.06-3a.75.75 0 0 1 1.06 0l10 10a.75.75 0 1 1-1.06 1.06l-10-10a.75.75 0 0 1 0-1.06Zm3.06-2a.75.75 0 0 0-1.06 1.06l10 10a.75.75 0 1 0 1.06-1.06l-10-10Z" /></svg>

[mtias]

Nice, I think this is a good step that would clarify things in global styles.

[jasmussen]

Nice, let's do this. The icon is fun too.

[noisysocks]

#63216 doesn't look near so marking this as punted to 6.8.

[mtias]

Noting that a part of this work was done in #65304.