Ubuntu – How to add a border around windows but keep the current theme

I seem to live my life in the terminal – many terminals in-fact.

I also love the Ambiance theme – but the new borderless windows does not provide the visual differentiation I'm used to (the same black windows appear to blend into each other).

Thus to my question – how do I add back the single pixel border for windows that is found in earlier Ubuntu versions?

I want to keep the Ambiance overall theme since this looks good and I dont want to swap to another theme.

Now I know that I can modify the Ubuntu Ambiance theme as follows:

mkdir ~/.themes
cp -r /usr/share/themes/Ambiance .themes

Then flipping between the theme options in the Settings – Appearance picks up changes in the theme in now in the .themes folder.

I presume I need to alter one of the .css files but I dont know which one.

I was also looking at this Q&A but the solutions there doesnt work for 14.04. I also looked at the community wiki but got rather lost trying to figure out which one applies to my situation.