In-app dark style switch
With the web adopting dark style support (thanks to support baked in now to Chrome, Firefox, and Safari across Windows, macOS, iOS, and Android), it might be worthwhile implementing an in-app switch for dark style in GNOME Web.
elementary OS and GNOME would like to have a well supported desktop-wide user preference for this that apps can opt into supporting, but that work is early on the elementary side and non-existent on the GNOME side. Instead, what GTK apps can do in the meantime is to have their own light/dark style preference in the app, requesting the GTK theme variant in response. Thanks to recent fixes, WebKitGTK already Does the Right Thing:tm: here and will support dark styles on websites if the app itself is requesting a dark GTK theme variant.
A caveat is that this preference should be designed and implemented in a way that will not conflict with a desktop-wide dark preference in the future—likely by just hiding the preference on a platform where that is supported and instead doing what the desktop is requesting.
I have some experience implementing this in my own Ephemeral browser, and can share some designs and background there if there is interest.