Cannot find the themes

Hi, I have a premium subscription.

I want to install the Asta theme.

I've been clicking about for an hour, I can't work out how to use this theme? It's not very intuitive at all for a newbie?!??! I chose Generate press because I didn't want to do coding, but this is impossible. I just want a big shiny button that says 'install theme'

Please can you give me a very detailed walkthrough, with pictures on how to get this theme onto my blog please.

Thanks
Becki