Corner radius of the cards ('md' is the login screen's default).
Color overrides, #rrggbb, by token. Absent means the login screen's default for that token.
The heading above the sign-in options; null for none.
Text on the story panel: 'light' text for a dark background, 'dark' for a light one.
How much of a split page the story panel takes ('balanced' is the stock 1.05 : 1).