You can use
sudo to gain root permissions.
Assuming you’ve downloaded the Paper theme* and extracted it to the folder
sudo cp -rf Paper /usr/share/themes
Hardy Theme by MariuxV?
If the theme you’re using is this one then it’s a little more complicated since there are multiple different kinds of themes and profiles in that file. The one that is probably useful for you is in the GTK folder inside the archive file.
So if you extracted the archive to
cd ~/Downloads/themes/Hardy-Mariux 2.0/GTK
sudo cp -rf "Hardy-Mariux 2.0" /usr/share/themes
To install any of the other customisations you’d need to look at the
readme.txt file in the top folder of the extracted archive.
I notice that the basic theme is also available as a .deb file.
Assuming you downloaded it to
sudo apt install hardy_theme_debian_package_by_mariuxv.deb
You could also right-click on the file in a file manager and choose Open With Software Install to install it using Ubuntu Software Center.
* Just an example—Paper theme has a PPA.