Seamless is a totally responsive parent theme meant to be a starting point for developers. It is built on top of the Hybrid Base theme so you are assured rock-solid php. Seamless takes care of the CSS. It fills in that blank stylesheet to provide you with a mobile-ready foundation geared toward modern web browsers (IE9 +). This will hopefully save you (freelancers and theme authors) a great deal of time in your theming endeavors.