I needed to install KDE temporarily in order to test something. Once I was done, I removed the KDE desktop environment package group and went back to using Gnome. Unfortunately, once I did that, Nautilus and several other Gnome applications had broken colour themes.
The following post explains the solution: https://www.reddit.com/r/gnome/comments/fn0ksn/broken_dark_theme_after_removing_kdeplasma/
Which I will summarize here by simply saying the following:
$ vi ~/.config/gtk-3.0/gtk.css
Comment out the @import 'colors.css'; line.
# @import 'colors.css';
And save it.
Restart Gnome and all should be back to normal:
alt+f2
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