Merge pull request #23706 from nextcloud/backport/23680/stable18

[stable18] Enable theming background transparency

Author: rullzer

apps/theming/lib/Controller/ThemingController.php

@@ -286,13 +286,17 @@ public function uploadImage(): DataResponse {
 			// either to big or are not progressive rendering.
 			$newImage = @imagecreatefromstring(file_get_contents($image['tmp_name'], 'r'));
 
+			// Preserve transparency
+			imagesavealpha($newImage, true);
+			imagealphablending($newImage, true);
+
 			$tmpFile = $this->tempManager->getTemporaryFile();
 			$newWidth = imagesx($newImage) < 4096 ? imagesx($newImage) : 4096;
 			$newHeight = imagesy($newImage) / (imagesx($newImage) / $newWidth);
 			$outputImage = imagescale($newImage, $newWidth, $newHeight);
 
 			imageinterlace($outputImage, 1);
-			imagejpeg($outputImage, $tmpFile, 75);
+			imagepng($outputImage, $tmpFile, 8);
 			imagedestroy($outputImage);
 
 			$target->putContent(file_get_contents($tmpFile, 'r'));