[stable19] Enable theming background transparency

[skjnldsv]

backport of #23680

[faily-bot]

🤖 beep boop beep 🤖

Here are the logs for the failed build:

Status of 34589: failure

mysql5.6-php7.2

Show full log

There were 2 warnings:

1) Test\Files\ViewTest::testRenameFailDeleteTargetKeepSource
Trying to configure method "writeStream" which cannot be configured because it does not exist, has not been specified, is final, or is static

2) Test\Files\ViewTest::testCopyFailDeleteTargetKeepSource
Trying to configure method "writeStream" which cannot be configured because it does not exist, has not been specified, is final, or is static

--

There were 2 failures:

1) Test\TagsTest::testAddTags
add() returned an ID <= 0
Failed asserting that 0 is greater than 0.

/drone/src/tests/lib/TagsTest.php:100

2) OCA\ShareByMail\Tests\ShareByMailProviderTest::testUserDeleted
Failed asserting that 543 is identical to 0.

/drone/src/apps/sharebymail/tests/ShareByMailProviderTest.php:869

[rullzer]

🐘