I want to give all my apps a light-gray interface without actually installing KDE or Kubuntu. I am using Ubuntu 14.04.
Is there a theme or something that can achieve this ?
The theme you are looking for it called Oxygen. However, you'd be better off using a theme such as Evolve:
http://satya164.deviantart.com/art/Evolve-GTK3-Theme-264780816
Oxygen would require some extra dependencies which you probably don't want.
To install Evolve:
sudo add-apt-repository ppa:satyajit-happy/themes
sudo apt-get update && sudo apt-get install evolve-gtk-theme