First, activate any other available theme (such as TwentySeventeen) under Appearance → Themes.
Then download this file and unzip it on your computer.
Once the file is unzipped, you should have a folder named thesis, which contains several different folders and files.
Access your server (preferably by using an FTP program), and navigate to the /wp-content/thesis/ folder.
Now, select all of the folders and files inside the thesis folder on your computer, and upload them to the /wp-content/thesis/ folder on your server.
Once that’s done, reactivate Thesis again under Appearance → Themes.
If these instructions didn’t work for you, here are the most common reasons why:
- You uploaded the
thesisfolder, rather than only its contents. - You uploaded to the
/wp-content/themes/thesis/folder instead.