button/    "Sign in with LatchID" buttons for partner login pages.
           latchid-button-{signin|continue|signup|icon}-{lg|md|sm}-{dark|light|brand}
           lg = 48px, md = 40px, sm = 32px tall. Dark on light backgrounds, Light on dark ones.
           Keep the wording and colours; match the size of other sign-in buttons.
