custom-background class appear in body class attribute even if theme doesn't support it

[wido]

There is an issue when the current theme doesn't support the 'custom-background' feature but the data ( if previously added ) is in the database.

To be clear (sorry for my english):

1. Add the theme support 'custom-background' for example:

   add_theme_support( 'custom-background', apply_filters( 'twentyseventeen_custom_background_args', array(
   		'default-color' => 'ffffff',
   		'default-image' => '',
   	) ) );
   

2. Select a color or a background image and check the body class attribute that has 'custom-background' class.

3. Remove the previous code and check for the body class. It's still there.

[wido]

Check for current theme support custom-background

[johnbillion]

@wido Thanks for the patch!

At a glance, this looks valid and the proposed patch makes sense.

[ocean90]

38168.2.diff​ updates the tests because the new test theme doesn't support custom-background by default.

[ocean90]

Hmm, the test doesn't work:

EF                                                                  2 / 2 (100%)

Time: 3.44 seconds, Memory: 84.00MB

There was 1 error:

1) Tests_Post_GetBodyClass::test_custom_background_class_is_added_when_theme_supports_it
Undefined index: wp-head-callback

/src/wp-includes/theme.php:2310
/src/wp-includes/theme.php:2266
/tests/phpunit/tests/post/getBodyClass.php:192

--

There was 1 failure:

1) Tests_Post_GetBodyClass::test_custom_background_class_is_not_added_when_theme_support_is_missing
Failed asserting that true is false.

/tests/phpunit/tests/post/getBodyClass.php:209

[Mte90]

patch refreshed with fix

[Mte90]

The issue was in theme.php, custom-header has the fix but not custom-background so now is fixed on the unit tests :-)

[pento]

Patch needs testing and review.

[SergeyBiryukov]

In 45088:

Customize: Only add custom-background to body_class() if the current theme supports custom background.

Props wido, swissspidy, ocean90, Mte90.
Fixes #38168.

[SergeyBiryukov]

In 45090:

PHPCS: Fix WPCS violation in [45088].

See #38168.