CSS is used to style and design HTML elements by applying rules that specify selectors, attributes, and values. CSS rules consist of a selector that identifies the element to style, an attribute to modify like color or size, and a value for the attribute. CSS can be applied on TFM by clicking elements to auto-generate rules and make live changes to previewed content. Examples demonstrate how to modify display, visibility, position, colors, fonts, backgrounds, and target specific app screens.