Change the theme of gi-docgen docs to match Adwaita

[AkshayWarrier]

As suggested by @bragefuglseth, we could try changing the styling of gi-docgen generated docs (such as changing bg/fg colors and setting font to Cantarell) to match the theme of Adwaita.

This might also be something we could consider doing for future docsets as well such as devdocs.

[orowith2os]

As an outside viewer, I think I prefer styling the app itself to fit gi-docgen, as is done right now:

And it has some visual consistency with the web version too, without getting into many styling issues.