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. This is the pane separator, and I want to change it so it matches the rest better (marked 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