Use get_background_color() in _custom_background_cb()

[greenshady]

When themes add a default background color via the custom-background feature, it isn't shown until the user saves it in the admin. The issue is that the _custom_background_cb() uses get_theme_mod( 'background_color' ) instead of get_background_color(), which will handle the default.

The attached patch should correct this.

Related: #20448