I have resolved the “are you sure you want to do this” issue using the links that you provided above.
I have tried to load the theme using the menu: Themes / Add new / Upload theme / Choose File / Install Now.
I get a new dialogue as follows:
Unpacking the package…
Installing the theme…
Destination folder already exists. /home/sesame53/public_html/wp-content/themes/jevelin/
Theme install failed.
Please can you help?