Inject dynamic value from back-end in freemarker code of keycloak

Question

Below is for keycloak 21. I have the below java class with a method that returns some dynamic data as "customCss" :

public class FetchCss {

    private static final String API_URL = "https://localhost:10191/mydata";

    public String fetchCustomCss() {
        RestTemplate restTemplate = new RestTemplate();

        // Construct API URL with query parameters to fetch active entry and custom_styles
        String apiUrl = API_URL + "?abcd=true";

        // Make GET request to fetch data
        ResponseEntity<List<StyleManagerDTO>> response = restTemplate.exchange(
                apiUrl,
                HttpMethod.GET,
                null,
                new ParameterizedTypeReference<List<StyleManagerDTO>>() {});

        // Extract customCss from response body
        List<StyleManagerDTO> styleManagerDTOList = response.getBody();
        if (styleManagerDTOList != null && !styleManagerDTOList.isEmpty()) {
            StyleManagerDTO styleManagerDTO = styleManagerDTOList.get(0);
            String customCss = styleManagerDTO.getCustomCss();
            System.out.println("customCss : " + customCss);
            return customCss;
        } else {
            return null;
        }
    }
}

i am trying to inject that data in the below freemarker code that I have for keycloak. How do I do it ?

<#import "templateLogin.ftl" as layout>
<@layout.registrationLayout displayMessage=!messagesPerField.existsError('username','password') displayInfo=realm.password && realm.registrationAllowed && !registrationDisabled??; section>
 
 <#assign customCss = customCss!>

<!-- Print the value of customCss for debugging purposes -->
<p>Custom CSS: ${customCss}</p>

<style type="text/css">
    ${customCss}
</style>

    
 .......

<Rest of the code>