I scanned the /home/ry/.themes/elementary-borderless/metacity-1/metacity-theme-1.xml file and didn't spot anything. Where else can I look?
If you still wanna know, look in /usr/share/themes/Ambiance/gtk-2.0/gtkrc – for example, for Ambiance theme. Search for style "scrollbar". Under the line engine "murrine", you can find such syntax:
It seems to use default background color (defined as bg_color at the beginning of the file) and you just use a shade. These values for shades result in a color close to white – I’ve changed the shades to change gtk-2 scrollbar to something like overlay one.
Remember, if you change the bg_color, all windows of all gtk-2 applications will be affected; so you may have better to try different values of shade. If you don't know, the values greater than 1 make the color lighter, and the smaller ones make darker.
Try in "/usr/share/themes//gtk-2.0/". The general file is gtkrc. Depending on how the theme is structured, it could include several other files. The naming convention appears to end in .rc, i.e. nautilus.rc or gedit.rc.
Read these: http://live.gnome.org/GnomeArt/Tutorials/GtkThemes/GtkScrollbar http://developer.gnome.org/gtk/stable/GtkScrollbar.html
You might find something from there.