Dark mode only works with Adwaita Dark themes.
With the new dark mode feature, using the Adwaita-dark and Adwaita-maia-dark themes will automatically make the browser use a dark theme for the home page and sites like Duckduckgo. However, themes like Arc-dark and the dark version of Matcha do not do this.
System Info: Manjaro Xfce x86_64 Gnome Web 3.36.1