To install themes system-wide, you would place them in /usr/share/themes, and icon themes would go in /usr/share/icons. But since you don't have root access, you can't write to those directories.

However, you can write files to your home directory. GTK themes can also be placed in ~/.themes (and icons go in ~/.icons). For example, if your username is "peter" you would place files in /home/peter/.themes.

These folders may not exist, and even if they do, you almost certainly won't be able to see them by default. Open your home folder and press [Ctrl]+[H] to display dot files - files and folders which start with a dot - these are hidden files under Linux. Now you should see several dot files, including stuff like .cache and .bash_logout. Don't remove these; they're there (and hidden!) for a reason. Instead, just right-click and Create New Folder and call it .themes.

Since you didn't specify the particular theme you're working with, I can't guide you on the exact structure they should be copied into the folder with. Feel free to edit your post, or look around in /usr/share/themes and mimic the structure with your own .themes directory.

Good luck!