To fix this (just for Chrome)
(See below for an answer that will work for all 32-bit applications, even Skype)
user140840's answer has the key: Accidentally running the 32-bit version of Chrome will lead to this problem. Fortunately, it is easy to fix as the Chrome repository is now already installed:
sudo apt-get remove google-chrome-stable:i386
sudo apt-get install google-chrome-stable
To also remove the unnecessarily installed 32-bit packages that you don't need anymore, run:
sudo apt-get autoremove
Then, you need to either restart Ubuntu or kill all running google-chrome processes:
Your settings will all still be there, and windows and menus should look correctly now.
To fix this permanently for all 32-bit applications (e.g. Skype)
You need to install the 32-bit versions of the GTK2 packages responsible (source):
sudo apt-get install gtk2-engines-murrine:i386 gtk2-engines-pixbuf:i386
Then close and restart the 32-bit application.
(both fixes tested and working on Ubuntu 14.04)