How do I change the cover, colors and fonts?

Open Design → Look. On the website it starts with the Opening, the sealed envelope guests press to open the invitation: see How do I make my invitation open like an envelope?. Choose a cover from the available collection, upload your own photograph, or leave the cover off. Adjust its framing, whichever cover you chose, then choose colors and fonts. A new photo opens on a layout that suits its shape, tall or wide, unless you have already picked a layout yourself; every layout stays on offer. Until you choose colors of your own, picking a photo also puts the page in Your photo's colors. A palette or mix you chose stays put when you change the photo.

The Color in Account → Appearance changes your own pages, never the invitation. See How do I change how my pages look?.

Check the preview, including the narrow phone layout. Every page and card shows the whole of the frame you set; a phone, an email and a link preview never crop it further. If a photo looks wrong somewhere, re-frame it once in the Design room. With Behind the title, the framer shows your own words on the picture as a guest will see them, and tells you how well the title will read over it. The words are treated so they read: a lighter ink from your colors, a soft edge, or a deeper wash. On the web they sit as they will on a phone; in the app, as they will on your device, once the framing you set has saved. Only a picture none of those can carry puts the words on your paper just under it.

Changing the design of a published event changes the guest page. An email already delivered is a separate copy, so do not rely on a design edit to update an invitation in someone's inbox.