Style the button border radius in the BIS modal window

We are using the new Klaviyo Back In Stock feature and I wanted to know if there was any way to add a border-radius to the submit button in the modal window? There are lots of other options that can be configured via the script, but nothing for this property.

        modal: {
        headline: "{product_name}",
        body_content: "Get notified via email when this product is back in stock.",
        email_field_label: "Enter your email address",
        newsletter_subscribe_label: "Add me to your email marketing list.",
        subscribe_checked: true,
        button_label: "Notify me",
        subscription_success_label: "Great, we will let you know when this is back in stock.",
        footer_content: '',
        additional_styles: "@import url('https://fonts.googleapis.com/css?family=Helvetica+Neue');",
        drop_background_color: "#3a3532",
        background_color: "#FFFFFF",
        text_color: "#3a3532",
        button_text_color: "#FFFFFF",
        button_background_color: "#3a3532",
        close_button_color: "#ccc",
        error_background_color: "#fcd6d7",
        error_text_color: "#C72E2F",
        success_background_color: "#3a3532",
        success_text_color: "#FFFFFF"
        }