Because Skype is a 32-bit only application it cannot use the default Oxygen theme in KDE on 64-bit systems, as it doesn't provide support for 32-bit applications. To fix this, download, extract and compile lib32-oxygen-qt4 by executing makepkg from the extracted location. After the package was created you can install it:

sudo pacman -U lib32-oxygen-qt4-4.11.15-1-x86_64.pkg.tar.xz

Now add /usr/lib32/kde4/plugins/ to your QT_PLUGIN_PATH environment variable by appending:

export QT_PLUGIN_PATH="$QT_PLUGIN_PATH:/usr/lib32/kde4/plugins/"

to .xprofile in your home directory (create the file if it not exists). Now just log out and log in again for the configuration to take effect. If you were using "Desktop settings" as a style in Skype's settings, now the application should match the style and colors of the desktop environment. Please note that this fix is working only for the Oxygen style - if you are using other styles, you'll need do find and install their 32-bit version to be able to use them in 32-bit applications.

Update: if this is not working, please check that you have all the required libraries dependencies for /usr/lib32/kde4/plugins/styles/oxygen.so:

ldd /usr/lib32/kde4/plugins/styles/oxygen.so | grep "not found"

 For me it was necessary to also install lib32-libxml2.