I have downloaded elementary-tweaks for Freya. I have also installed a few themes using PPA. What if I have a theme in a file and I want to include it in the gallery of the themes? How can I add it manually without having to use the PPA?
There are two ways:
Copy the theme to the system wide theme folder
sudo cp -R [theme-folder] /usr/share/themes
Copy the theme to your users local theme folder
cp -R [theme-folder] ~/.themes
If your local theme folder doesn't exist yet, you will have to create it first:
mkdir -p ~/.themes
It's not recommended to use elementary-tweaks as it has been known to break systems. Instead use this command:
gsettings set org.gnome.desktop.interface gtk-theme "CoolestThemeOnEarth"
However, if you want to, you can use elementary Tweaks to do the same thing.