Remove Ghost link from default theme

1. Login to your Ghost admin dashboard.
2. Go to Design > Installed theme.
3. Download your active Casper 3.0 theme.
4. Unzip the theme folder and Open default.hbs file.
5. Remove the following code and save

<a href="https://ghost.org" target="_blank" rel="noopener">Ghost</a>

6. Zip and upload the folder. Activate the theme.