[Feature] give gnome colors to firefox "about:" pages and popup dialogs

[erlefloch]

Is your feature request related to a problem? Please describe.
I feel that the default colors of firefox internal pages don't match with adwaita colors, especially with the dark theme on.

Describe the solution you'd like
I would like the firefox gnome theme to change the default colors of firefox, on "about:" pages like about:settings, info pages like the "Secure Site Not Available" page, and on popup dialogs like the printing dialog.

Describe alternatives you've considered
Maybe it is possible to changing these settings manually in about:config, but it is hard to figure out what setting changes what feature.

Additional context
screenshots:

[rafaelmardojai]

Duplicated of #244.