Themes: Prevent unneeded database updates in `wp_get_custom_css_post()`.

When a custom header image was set but custom CSS was not, `wp_get_custom_css_post()` was generating an UPDATE query on every frontend request.

In theme options the header image meta data is stored as an object. In `update_option()` this hits an edge case as the resource IDs of the old and new values never match.

This changes the logic of `wp_get_custom_css_post()` to ensure `set_theme_mod()` is only called when the custom CSS has changed.

Props bradyvercher, helen.
Fixes #38866.

Built from https://develop.svn.wordpress.org/trunk@39338


git-svn-id: https://core.svn.wordpress.org/trunk@39278 1a063a9b-81f0-0310-95a4-ce76da25c4cd