diff --git a/apps/theming/lib/Capabilities.php b/apps/theming/lib/Capabilities.php
index d10002c9deae8..f0c4d2663622c 100644
--- a/apps/theming/lib/Capabilities.php
+++ b/apps/theming/lib/Capabilities.php
@@ -69,7 +69,7 @@ public function __construct(ThemingDefaults $theming, Util $util, IURLGenerator
 	 * @return array
 	 */
 	public function getCapabilities() {
-		$backgroundLogo = $this->config->getAppValue('theming', 'backgroundMime', false);
+		$backgroundLogo = $this->config->getAppValue('theming', 'backgroundMime', '');
 		$color = $this->theming->getColorPrimary();
 		return [
 			'theming' => [
@@ -82,10 +82,10 @@ public function getCapabilities() {
 				'color-element-bright' => $this->util->elementColor($color),
 				'color-element-dark' => $this->util->elementColor($color, false),
 				'logo' => $this->url->getAbsoluteURL($this->theming->getLogo()),
-				'background' => $backgroundLogo === 'backgroundColor' || ($backgroundLogo === false && $this->theming->getColorPrimary() !== '#0082c9') ?
+				'background' => $backgroundLogo === 'backgroundColor' || ($backgroundLogo === '' && $this->theming->getColorPrimary() !== '#0082c9') ?
 					$this->theming->getColorPrimary() :
 					$this->url->getAbsoluteURL($this->theming->getBackground()),
-				'background-plain' => $backgroundLogo === 'backgroundColor' || ($backgroundLogo === false && $this->theming->getColorPrimary() !== '#0082c9'),
+				'background-plain' => $backgroundLogo === 'backgroundColor' || ($backgroundLogo === '' && $this->theming->getColorPrimary() !== '#0082c9'),
 				'background-default' => !$this->util->isBackgroundThemed(),
 				'logoheader' => $this->url->getAbsoluteURL($this->theming->getLogo()),
 				'favicon' => $this->url->getAbsoluteURL($this->theming->getLogo()),
diff --git a/apps/theming/lib/ImageManager.php b/apps/theming/lib/ImageManager.php
index 263a242f7f026..6ba7df3ffa942 100644
--- a/apps/theming/lib/ImageManager.php
+++ b/apps/theming/lib/ImageManager.php
@@ -104,9 +104,9 @@ public function getImageUrlAbsolute(string $key, bool $useSvg = true): string {
 	 */
 	public function getImage(string $key, bool $useSvg = true): ISimpleFile {
 		$pngFile = null;
-		$logo = $this->config->getAppValue('theming', $key . 'Mime', false);
+		$logo = $this->config->getAppValue('theming', $key . 'Mime', '');
 		$folder = $this->appData->getFolder('images');
-		if ($logo === false || !$folder->fileExists($key)) {
+		if ($logo === '' || !$folder->fileExists($key)) {
 			throw new NotFoundException();
 		}
 		if (!$useSvg && $this->shouldReplaceIcons()) {
diff --git a/apps/theming/lib/ThemingDefaults.php b/apps/theming/lib/ThemingDefaults.php
index 174491beb2213..66a961ccb3cbd 100644
--- a/apps/theming/lib/ThemingDefaults.php
+++ b/apps/theming/lib/ThemingDefaults.php
@@ -223,7 +223,7 @@ public function getColorPrimary() {
 	 * @return string
 	 */
 	public function getLogo($useSvg = true): string {
-		$logo = $this->config->getAppValue('theming', 'logoMime', false);
+		$logo = $this->config->getAppValue('theming', 'logoMime', '');
 
 		$logoExists = true;
 		try {
@@ -307,17 +307,17 @@ public function getScssVariables() {
 		$variables['image-login-plain'] = 'false';
 		$variables['image-dashboard-plain'] = 'false';
 
-		if ($this->config->getAppValue('theming', 'color', null) !== null) {
+		if ($this->config->getAppValue('theming', 'color', '') !== '') {
 			$variables['color-primary'] = $this->getColorPrimary();
 			$variables['color-primary-text'] = $this->getTextColorPrimary();
 			$variables['color-primary-element'] = $this->util->elementColor($this->getColorPrimary());
 		}
 
-		if ($this->config->getAppValue('theming', 'backgroundMime', null) === 'backgroundColor') {
+		if ($this->config->getAppValue('theming', 'backgroundMime', '') === 'backgroundColor') {
 			$variables['image-login-plain'] = 'true';
 		}
 
-		if ($this->config->getAppValue('theming', 'dashboardBackgroundMime', null) === 'backgroundColor') {
+		if ($this->config->getAppValue('theming', 'dashboardBackgroundMime', '') === 'backgroundColor') {
 			$variables['image-dashboard-plain'] = 'true';
 		}
 
diff --git a/apps/theming/lib/Util.php b/apps/theming/lib/Util.php
index 3addda9142200..e757d100a1b15 100644
--- a/apps/theming/lib/Util.php
+++ b/apps/theming/lib/Util.php
@@ -249,7 +249,7 @@ public function isAlreadyThemed() {
 	}
 
 	public function isBackgroundThemed() {
-		$backgroundLogo = $this->config->getAppValue('theming', 'backgroundMime',false);
+		$backgroundLogo = $this->config->getAppValue('theming', 'backgroundMime', '');
 
 		$backgroundExists = true;
 		try {