Customizing the button style is done differently depending on the JS framework you use.
Change the button to light mode by setting theme as light. Default is dark.

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Customizing the button UI in React

Add custom styles to the CrossmintPayButton component using CSS. See the example of the React SDK below and note the className attribute:
import { CrossmintPayButton } from "@crossmint/client-sdk-react-ui";

export default function Mint() {
  return (
    <div>
      <CrossmintPayButton
        projectId="_YOUR_PROJECT_ID_"
        collectionId="_YOUR_CLIENT_ID"
        className="xmint-btn"
      />
    </div>
  );
}