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It's possible to override the style on individual instances of content in a few different ways.
At the level of a group, it is possible to overlay the style for all content within that group. With a group selected, click Style, check Override default style, and finally Edit style override... to create edits that apply to that group alone.
Question container styles are similar to CSS classes. They are defined in Questions within the style editor, and overlay the default style that is applied to all questions.
Once you define container styles, any question can use any of the container styles.
These are helpful to make different kinds of questions visually distinct: a common pattern is to create an optional question class that is used for optional questions, with left border and margin to differentiate it from non-conditional questions.