Either add elementary stylesheet or build with elementary SDK

[danirabbit]

What Happened

Epiphany is back to using Adwaita again as the stylesheet

Expected Behavior

Epiphany should use the elementary stylesheet. We have a couple of options here:

• We could add the stylesheet itself to the flatpak
• We could use the elementary Sdk. However, I'm not sure that Epiphany 40 will build against elementary Sdk since it's currently based on the GNOME 3.38 Sdk.

[alice-mkh]

It bundles the stylesheet and icons on Flathub: https://github.com/flathub/org.gnome.Epiphany/blob/master/org.gnome.Epiphany.json#L19-L99

[Marukesu]

The manifest on this repo runs with --runtime=io.elementary.Platform//0.1.0, same for the Devel.