I have 14.04, but
gedit over there has an option to change color scheme --- the preferences menu is quite hidden (at least in gnome-shell), you have to go on the panel menu to find it:
(will be different in Unity), and then:
and all your color scheme will change. In the shot above, I choose "kate".
If you want to change just the color of the highlight line (for example), you should edit the style; in principle that should be easily done by changing the file
(reference here) and change the line
<style name="current-line" background="#EEF6FF"/>
to your liking. Unfortunately, it doesn't work --- if anyone know why please chime in.