diff --git a/apps/theming/lib/Themes/DarkHighContrastTheme.php b/apps/theming/lib/Themes/DarkHighContrastTheme.php
index e7f52bb047531..fdb9e27fe5d35 100644
--- a/apps/theming/lib/Themes/DarkHighContrastTheme.php
+++ b/apps/theming/lib/Themes/DarkHighContrastTheme.php
@@ -57,6 +57,7 @@ public function getCSSVariables(): array {
 		$colorMainBackground = '#000000';
 
 		$variables['--color-main-background'] = $colorMainBackground;
+		$variables['--color-main-background-translucent'] = 'rgba(var(--color-main-background-rgb), .1)';
 		$variables['--color-main-text'] = $colorMainText;
 
 		$variables['--color-background-dark'] = $this->util->lighten($colorMainBackground, 30);
diff --git a/apps/theming/lib/Themes/HighContrastTheme.php b/apps/theming/lib/Themes/HighContrastTheme.php
index 561f7dc9479f8..a9ba6da42a931 100644
--- a/apps/theming/lib/Themes/HighContrastTheme.php
+++ b/apps/theming/lib/Themes/HighContrastTheme.php
@@ -54,6 +54,7 @@ public function getCSSVariables(): array {
 		$colorMainBackground = '#ffffff';
 
 		$variables['--color-main-background'] = $colorMainBackground;
+		$variables['--color-main-background-translucent'] = 'rgba(var(--color-main-background-rgb), .1)';
 		$variables['--color-main-text'] = $colorMainText;
 
 		$variables['--color-background-dark'] = $this->util->darken($colorMainBackground, 30);