Fix issue with quiet zone around QR code (#9243) (#9257)

(cherry picked from commit b651b1c)

Co-authored-by: Janne Mareike Koschinski <[email protected]>

Author: RiotRobot

res/css/views/dialogs/_AppDownloadDialog.pcss

@@ -29,6 +29,9 @@
             gap: $spacing-16;
 
             .mx_QRCode {
+                /* intentionally hardcoded color to ensure the QR code is readable in any situation */
+                background: #ffffff;
+
                 padding: $spacing-24;
                 border: 1px solid $quinary-content;
                 border-radius: 4px;