There are 4 parameters you should pass to the iframe URL:
c: This is the generated API key for the iframe, without this, the iframe will block the connection from your site.
Theme: You can switch between "dark" and "light" themes. If the value is invalid, it defaults to the light theme. For example "theme=dark".
Radius: You can set the border radius of the elements such as inputs, buttons, etc. The number can be set in "px", "rem", or any other CSS value. For example "radius=8px".
Colour: You can set the primary color of the button elements, this should be a hexadecimal value but without the "#". For example "color=ffaf20".