yes, a membership gives you access to premium themes and plugins... currently, there is only 1 premium theme, the mobile responsive theme you referred to, though a second mobile responsive theme will be released soon...  there are 20 or 30 premium plugins that extend the functionality of simple press...

alternatively, we do offer a custom theme (and plugin) development service where we can customize a theme to your specifications...  it can be just colors or different layout or functionality changes...  if you want to discuss a custom theme, please use our contact form and let us know what you want and we will get back to you to discuss specifics and work a quote for you...