Kind of difficult to categorize this tweak. It is for digikam (KDE) but it is for solving an issue with digikam under GTK...
After installing digikam on my Ubuntu MATE I had problems making digikam follow the rest of the look and feel of my MATE desktop.
I use the Arc Dark theme but in digikam I only had the default color scheme to choose from.
I made a new *.colors-file which followed the Arc Dark colors.
Simply extract the archive and put "ArcDArkGTK.colors" in "~/.kde/share/apps/color-schemes/" and start/restart digikam and pick ArcDarkGTK from Settings-> Themes menu.