Why doesn't Skype theme match my Desktop Environment theme?

I have found a solution for this problem: The odd look is caused because of the fact that skype is a 32 bit application, so if you are on a 64-bit system (which you are, because you suffer from this problem) you need to install additional 32-bit libaries to solve this issue. Just run:

sudo apt-get install ia32-libs-gtk

restart skype an everything will look integrated! (This libary will use 144 MB of additional space!)

Hope that works for you too!

In 14.04 (Trusty Tahr) 15.10 (Wily Werewolf) 16.04 (Xenial Xerus)

On Trusty (and above), you don't need the entire i386 lib stack, just install:

sudo apt-get install gtk2-engines-murrine:i386 gtk2-engines-pixbuf:i386