dashboard: Fix dark theme detection

When system default color theme is selected for theming,
the enabledThemes array is empty or just contains one entry 'default',
in this case the color theme has to be retrieved from the browser.

Signed-off-by: Ferdinand Thiessen <rpm@fthiessen.de>

apps/dashboard/src/helpers/getBackgroundUrl.js

@@ -27,7 +27,9 @@ import prefixWithBaseUrl from './prefixWithBaseUrl'
 
 export default (background, time = 0, themingDefaultBackground = '') => {
 	const enabledThemes = window.OCA.Theming.enabledThemes
-	const isDarkTheme = enabledThemes.join('').indexOf('dark') !== -1
+	const isDarkTheme = enabledThemes && (enabledThemes.length > 1 || enabledThemes[0] !== 'default')
+		? enabledThemes.join('').indexOf('dark') !== -1
+		: window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches
 
 	if (background === 'default') {
 		if (themingDefaultBackground && themingDefaultBackground !== 'backgroundColor') {