I'm aware of that; but can people use the Sandbox skins provided and put presentational stuff in the custom CSS, or do they need to start from scratch? It sounds like the former, with the latter as an option for people who want to handle positioning themselves, but it isn't being made very clear.
OK, let's try again. This is how I'm assuming stylesheets for Sandbox will work:
basic stylesheet (same for everyone using the theme, sets font sizes & margins and stuff, cannot be edited)
+ optional 'skin' stylesheet to control the number and position of columns (selected by user in presentation panel, available to all users, cannot be edited)
+ optional custom CSS (written by user or copy/pasted from somewhere else, available to paid users only)
(To be pedantic, the web developer extension suggests that the skin loads before the basic stylesheet, but I've kept this order as it makes more sense conceptually)
Most other themes:
basic stylesheet
+ optional custom CSS
Because the custom CSS is being loaded after the basic stylesheet, it can override its settings. But if you leave a property alone in the custom CSS, it will default to the basic stylesheet (fairly obvious to anyone who already knows about CSS, but it may not be quite as obvious to everyone else).
Am I missing anything?