I make all this questions to try to get some rules :)
So let me recap:
-Create a layout.css with all properties
-If we have 2 skins: create 2 new files with only the specific properties for each skin (and remove them from layout.css)
-icons.css in case we have icons (we will soon have an icon font to replace this)
-responsive.css for rotation (in case styles are different from portrait to landscape) and also for modifying styles in larger devices (e.g tablet)