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The signature can be customized under "Settings" → "Identities" → "Your Identity" in the "Signature" field.
Additional options for the signature can be found under "Settings" → "Composing Message" → "Signature Options"
Optional: HTML formatting in the signature
To get more options in formatting, the signature can also be captured in HTML by clicking on the icon in the top left corner:
Additionally you have to activate the option "always" under "Preferences" → "Composing Messages" → "Compose HTML messages":
