Support Foliate theming for dark

[dxps]

Hello again!

This theme is something that I love and I'm using it for months, which is mainly a confirmation to me that I have what I like and need. :blush:

One thing I was hoping to have as well is to properly handle Foliate theming, at least for dark. I'm not sure is this app specific or more of a GTK one.

By default, it uses a "header bar" option, so the end result looks like this:

But if you mouse hover that area or click on the contents (that is a toggle to show/hide that header bar and footer), it looks like this:

If I'm using Pop!_OS Dark Theme, it shows a dark flavor of it, which is what I'm chasing for.

Any advice, please :grey_question: I'll take a look on what that Pop!_OS theme has, specific to Foliate.

Thanks!