fix: move dark mode and transparent body hook into renderer

These hooks are only necessary for the document and simple renderer. The slideshow handles theming itself.

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

frontend/src/components/render-page/hooks/use-transparent-body-background.ts

@@ -0,0 +1,16 @@
+/*
+ * SPDX-FileCopyrightText: 2023 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { useEffect } from 'react'
+
+/**
+ * Makes the HTML body transparent.
+ */
+export const useTransparentBodyBackground = () => {
+  useEffect(() => {
+    document.body.classList.add('bg-transparent')
+    return () => document.body.classList.remove('bg-transparent')
+  }, [])
+}