There is no need to add square brackets - "[" and "]" when you put your data to this line:
theme get --password=[your-api-password] --store=[your-store.myshopify.com] --themeid=[your-theme-id]
You need to paste the following:
theme get --password=Your-api-password --store=Your-store.myshopify.com --themeid=Your-theme-id
Hope this helps.
I'm not sure if this will help anyone, but i was having this issue, while trying to grab my theme id using the --list command, i noticed that the shorthand for store, -s, didn't work, i had to use all the long commands so --password and --store instead of -p or -s. not sure why it worked one way and not the other but that was the trick for me