Are there any GTK3 themes that work properly in Unity-2D? All that I've tried have "button glitches"

After further testing, at the moment the answer to my question seems to be no. It appears that the only included Ubuntu Ambiance and Radiance have the ability to render properly when maximized for the reason stated in my previous comment. This has been filed as bug #984818 at Launchpad.

Until this problem is fixed with the Unity-2D panel it not possible for any third party theme to work properly.

I have recently experienced the same problem in my netbook with unity 2d, and after searching the internet and failing in finding a solution for this, I had an idea which actually worked perfectly.

If you want the buttons of your custom theme working as they should just go to /usr/share/themes and rename the folder of your custom theme to Ambiance, you can also rename the original however you want, for example "Real Ambiance" to remember where it is in the case you want to revert the changes. Afterwards restart your computer and it should work, at least it did for me.

I have days in the linux ecosystem and therefore I have no idea why this might be, I just followed my frustration-leaded instincts, and thought that I should post the workaround here for the people that are suffering from this glitch.

Hope it helps.