allow to override prefers-color-scheme via url param for guests

using ?force-color-scheme=light should make the theme light and ?force-color-scheme=dark should make it dark.
Useful for iframes to match a website theme and to send a specific theme to a guest.

apps/accessibility/src/accessibilityoca.js

@@ -24,8 +24,19 @@
 import { loadState } from '@nextcloud/initial-state'
 
 OCA.Accessibility = loadState('accessibility', 'data')
-if (OCA.Accessibility.checkMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {
-	// Overwrite the theme for Guests based on the prefers-color-scheme
+// Force a specific theme for guests based on force-color-scheme param
+var srv_url = new URL(window.location.href)
+var force_color_scheme = srv_url.searchParams.get("force-color-scheme")
+if(force_color_scheme !== null) {
+	if(force_color_scheme=="dark") {
+		OCA.Accessibility.theme = 'dark'
+	}
+	else {
+		OCA.Accessibility.theme = 'light'
+	}
+}
+else if (OCA.Accessibility.checkMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {
+	// Overwrite the theme for guests based on the prefers-color-scheme
 	OCA.Accessibility.theme = 'dark'
 }