While there are a ton of free WordPress themes out there, most of the ones I run across do not have the polish that a lot of the premium ones do. Pytheas is different. It’s a really quality theme that’s simple and elegant, and of course – responsive. This is a free theme I would be willing to use on one of my own sites.
It’s offered free of charge by WP Explorer, but you do have to register for their site to be able to download it (registration is free). There’s even a video that gives you a good overview of the theme.
Pytheas uses the built in Theme Customizer options that come with WordPress avoiding a lot of unnecessary clutter.