The two solutions above did not work for me. This is my solution:
Open the terminal.
Open the document geany.css as root in an editor (for example Bluefish) like this:
sudo bluefish /usr/share/geany/geany.css
Enter the password; Enter.
Search for: status-changed.
In the line below you can change the color.
Save the document.
Install pdf-tools from melpa and enable pdf-view-midnight-minor-mode in pdf-view-mode. The default color for midnight mode is light grey on dark blue but this can be changed by configuring pdf-view-midnight-colors, eg.
(setq pdf-view-midnight-colors '("#000000" . "#dcdcdc"))
PS: It works on scanned copies too.
so basically you can do sudo gedit /usr/share/themes/Arc/gtk-2.0/gtkrc and just change the color you want.
this will change the task bar:
Unfortunately it also has other effects like this one:
So there is a weird Arc problem that shows for some reason shows a dark sidebar which is just stupid, but now the selection color here changes as well to purple
I couldn't find "Arc light" in the repositories but plain Arc has the same issue you describe.
The file you need to edit is /usr/share/themes/Arc/gtk-2.0/panel.rc. Change all instances of @selected_bg_color in this file to
"#2A4286", a slightly darker shade of blue, or any other distinctive color of your choice.
Note that you'll need to use sudo -H ...