By way of illustration, here's 'Likebook for Facebook' in action today on my Windows 10 Alcatel IDOL 4 Pro:
With dark theme turned on, 98% of content is indeed 'dark', though some panels escape. Still Likebook's getting these updates, so why wouldn't the theming improve further? Note also the way I've maxxed out the screen real estate by turning off the top status bar and bottom controls, you're left with just the horizontal '...' Here's my playing a classic Allo Allo clip inline. So funny!
Tapping this '...' menu brings up save and link copying options, plus useful Refresh, Top (of page), and Home controls; (right) the Settings pane is comprehensive - complete control over layout, even if Facebook doesn't want you to have it!
The 'mobile' web view of Messenger shows through too - useful to check you haven't been sent any messages, I guess. Though no one I know really uses Messenger, so it's hard to tell; (right) proving you can post content too, here adding three photos to a post, which you can find here.
You can grab this in the Store here.
PS. See also my overview of Facebook apps earlier in the year.