Enable theming background transparency

[skjnldsv]

Fix#23649

Example with colour #9F40C9

Before	After

[faily-bot]

🤖 beep boop beep 🤖

Here are the logs for the failed build:

Status of 34541: failure

sqlite

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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 was 1 failure:
1) Test\Files\Cache\UpdaterLegacyTest::testWrite
Failed asserting that 1603650101 is equal to 1603650102 or is greater than 1603650102.
/drone/src/tests/lib/Files/Cache/UpdaterLegacyTest.php:118

mysql8.0-php7.4

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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 was 1 failure:
1) Test\Files\Storage\Wrapper\QuotaTest::testFreeSpaceWithUnknownDiskSpace
Failed asserting that -1 matches expected 6.
/drone/src/tests/lib/Files/Storage/Wrapper/QuotaTest.php:92

[MorrisJobke]

Tested and works 👍

[skjnldsv]

/backport to stable19

[skjnldsv]

/backport to stable20

[skjnldsv]

/backport to stable18

[backportbot-nextcloud]

The backport to stable19 failed. Please do this backport manually.

[backportbot-nextcloud]

The backport to stable18 failed. Please do this backport manually.