This compares to how you often change your desktop wallpaper to freshen up the operating system's look :)
So, to install a FireFox theme
Go to "Add-ons", find the theme you need there and install it. Besides that, there are alternative installation methods, but this one is the main and most effective.
Details about managing themes in FireFox can be found in the official FireFox documentation available on the FireFox website. By the way, after installing the theme you will need to restart FireFox for the changes to take effect.
That's it, good luck!