Comment 7 for bug 348500

You need to add something like this to your theme file:

<style name="draw-spaces" foreground="tinted_grey"/>

Look in the cobalt.xml file in /usr/share/gtksourceview-2.0/styles for an example of a theme which does this. More info here: http://www.codeistry.com/blog/2009/05/choosing-syntax-highlighting-colours-for-the-draw-spaces-gedit-plugin/