Please advise how to download Casper theme in order to insert disqus

Using Ghost
2

If you’re using the Ghost PRO (hosted version), you can download and upload the theme as follows:

  1. Go to Settings > Design > Change Theme
  2. Open the Advanced dropdown
  3. Click the three dots (…) next to your theme name and click Download
  4. After you make the changes, zip them up and upload the zip file by clicking the Upload theme button on the same page.

That said, Ghost now supports native comments, so you might not need to use Disqus.

Hope it helps.
Akshay