All themes are only covering half the toolbar on 4k screen.
I have a 4k screen for my Windows 10 machine and my new laptop that is running linux has a 4k screen. I am having the same problem on both of them. When I make Firefox full screen I the left third of the tool bar does not display a theme and only shows the backgound color. This happens with all themes. Basicly the themes aren't scaling across the full window try uploading a screen shot but it would never finish uploading, so after an hour of it sitting there I gave up.
PS.....even themes advertived as 4k theme are having the same issue.
Zgjidhje e zgjedhur
As said these are not really Themes but background wallpaper images.
The real Themes are listed in https://addons.mozilla.org/firefox/complete-themes/
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Lightweight themes UI images are spec'd to be 3000px wide × 200px high, so it figures that the image would only cover 3/4 of the screen width. That spec dates to 2009, so 4K wasn't around.
Zgjidhja e Zgjedhur
As said these are not really Themes but background wallpaper images.
The real Themes are listed in https://addons.mozilla.org/firefox/complete-themes/