Theming Gnome Shell
Is encouraged to not theme Gnome Shell; designers and developers of Gnome Shell try to provide the best experience with the default theme, and theming Gnome Shell can cause wrong behaviours.
If you want to theme it anyway, you need gnome-tweak-tool which should be provided on your distribution. Install it:
Ubuntu/Debian and .deb systems
sudo apt-get install gnome-tweak-tool
sudo dnf install gnome-tweak-tool
Then go to https://extensions.gnome.org/extension/19/user-themes/ with Firefox (it only works with Firefox) and install that extension named "user themes".
Download a theme from websites like http://gnome-look.org and put it on ~/.themes folder (you can use /usr/share/themes to use a theme system wide)
Open Tweak tool application you installed previously and in the Appearance tab select the themes you want.