No no, it's a theme that already exists that you can find in customise style. Organise -> customise style -> box on the right side of the screen which says 'krytella's current theme': click 'select new theme'. Then in the search field for themes put 'mobility'. It comes in two colou schemes, a beige and a blue, but you can change those colours on the previous page in the customise your theme section.

Re: site skin, that only affects the look of the dw pages that aren't your journal, like the home page. The standard is grey and red, but there are other colour schemes and looks. Your journal theme will affect your journal and what your reading page looks like. (And in settings you can make it so that you view other people's entry pages in your journal style instead of their own or the site skin.)