Install a theme that isn't in the skeletons

Hi, I can't remember where to look to find out how to install a theme that isn't in the skeletons, for example the "landed" theme... I'd like a hint.

I asked this question because I've lost some of the notices I translated into French, for which I apologize.
I will exercise more restraint before asking other questions in the future, at least simplistic ones like this one.
Thank you very much anyway, as this reply is only useful. jef