Alternate CSS Style Selector (ACSS for short) provides an interface to choose from alternate stylesheets which are provided by some sites. When an alternate stylesheet is available an icon will appear in the omnibox, which you can click to see a list of styles to choose from. After choosing a style, ACSS will remember it until you choose another.
The "Base (ACSS)" style is all the stylesheets without a title or rel=alternate; the other styles are "Base (ACSS)" plus some extra. "None (ACSS)" disables all styles.