A look is the style of the invite your guests open. There are eight: Folio, Marquee, Reserve, Atlas, Bento, Aurora, Poster and Lumen. Every event starts in Folio.
Pick one in the Look row when you make the event, or in Edit later. Each shows as a small swatch with its name, and the preview shows the invite in it.
A look changes the invite page, the card a shared link opens as, and the poster. It doesn't change your emails, and it never changes what a guest does: the answer sits in the same place, in the same words, in every look.
Aurora has four colors to choose from: Sunset, Lagoon, Orchid and Ember. If you don't pick one, the color comes from the event's name, so renaming the event can change it.
Add a photo and most looks build themselves around it. Without one they hold up on type alone.
Changing the look after the invite has gone out changes it for everyone who already has it, including people who have answered. Edit warns you before you save, with how many people have it. A new Aurora color does the same, without the warning.