"Manage Themes" Download new theme

Hello-
I’m new to hosting. On my site under “Manage Themes” “You only have one theme enabled for this site right now” greets me, with an option to enable (none there) or install, which links to a search box. I go there, I type in a theme, using “author”,”term” or “tag” search option, and receive the message, “An Unexpected HTTP Error occurred during the API request. Try again. I do, nada” I would like to practice with different themes and am unsure how to progress. Can anyone knowledgeable in this help me,please?