IONOS(Theming): fix the background color in darkmode

partial revert of 647f4c4

Signed-off-by: Tatjana Kaschperko Lindt <kaschperko-lindt@strato.de>

Author: tanyaka

apps/theming/lib/Themes/IonosTheme.php

@@ -234,7 +234,7 @@ public function getCSSVariables(): array {
 
 			// used for different active/hover/focus/disabled states
 			'--color-background-hover' => 'light-dark( var(--ion-color-blue-b1), var(--ion-color-blue-b8))',
-			'--color-background-dark' => $this->util->darken($colorMainBackground, 7),
+			'--color-background-dark' => 'light-dark( ' . $this->util->darken($colorMainBackground, 7) . ', var(--ion-color-blue-b6))',
 			'--color-background-darker' => $this->util->darken($colorMainBackground, 14),
 
 			'--color-background-plain' => $ionColorMainBackground,