I am currently messing around with Ambiance, trying to give Nautilus a dark sidebar (because I think it looks much better that way, especially with the current look having the dark-colored breadcrumbs clashing horribly with the light-colored sidebar). I have zero experience and knowledge of how to create GTK+ themes, and I couldn't find any documentation online, so I just made a copy of the folder for Ambiance under /usr/share/themes, renamed it "Ambiance Dark Sidebar" and just started messing with color values.

As shown below, I found the value in nautilus.css needed to be tweaked to create the dark sidebar, but there is still one part that stubbornly stays light gray that I want to change so it matches the rest better (I'm not sure what the proper terminology is here, so I just provided a picture and marked it in red).

Does anyone know what I need to do to change the color of this part so it matches the rest of the sidebar better? I already know from seeing themes like Adwaita Dark that this should be possible, but even after poking around in that I didn't find anything that seemed to help.

Here are the contents of the files I modified in the theme folder Ambiance Dark Sidebar, stored alongside Ambiance in /usr/share/themes:

• index.theme
• gtk-3.0/apps/nautilus.css