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.
פתרון נבחר
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.
פתרון נבחר
As said these are not really Themes but background wallpaper images.
The real Themes are listed in https://addons.mozilla.org/firefox/complete-themes/