Add backgroundImage to the theme json docs for appearanceTools #86532

Job	Run time
JavaScript (16)	9m 55s
Compute previous WordPress version	3s
Mobile	6m 22s
PHP coding standards	27s
PHP 7.0 on ubuntu-latest	7m 11s
PHP 7.0 multisite on ubuntu-latest	6m 48s
PHP 7.1 on ubuntu-latest	7m 11s
PHP 7.1 multisite on ubuntu-latest	7m 15s
PHP 7.2 on ubuntu-latest	6m 51s
PHP 7.2 multisite on ubuntu-latest	6m 56s
PHP 7.3 on ubuntu-latest	7m 49s
PHP 7.3 multisite on ubuntu-latest	7m 1s
PHP 7.4 on ubuntu-latest	6m 51s
PHP 7.4 multisite on ubuntu-latest	6m 40s
PHP 8.0 on ubuntu-latest	7m 54s
PHP 8.0 multisite on ubuntu-latest	6m 57s
PHP 8.1 on ubuntu-latest	6m 58s
PHP 8.1 multisite on ubuntu-latest	8m 53s
PHP 8.2 on ubuntu-latest	7m 7s
PHP 8.2 multisite on ubuntu-latest	7m 3s
PHP 7.0 (WP 6.2.3) on ubuntu-latest	6m 41s
PHP 7.4 (WP 6.2.3) on ubuntu-latest	6m 38s
PHP 8.2 (WP 6.2.3) on ubuntu-latest	8m 46s
PHP	7s
	2h 34m 24s

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