curl --request PATCH \
  --url https://{your-subdomain}.neetoform.com/api/external/v2/forms/{form_id}/theme \
  --header 'Content-Type: application/json' \
  --header 'X-Api-Key: <x-api-key>' \
  --data '
{
  "theme_id": "3b0d5c1e-2f7a-4c4e-9d1b-6a8f2e7c9b10"
}
'

const options = {
  method: 'PATCH',
  headers: {'X-Api-Key': '<x-api-key>', 'Content-Type': 'application/json'},
  body: JSON.stringify({theme_id: '3b0d5c1e-2f7a-4c4e-9d1b-6a8f2e7c9b10'})
};

fetch('https://{your-subdomain}.neetoform.com/api/external/v2/forms/{form_id}/theme', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

import requests

url = "https://{your-subdomain}.neetoform.com/api/external/v2/forms/{form_id}/theme"

payload = { "theme_id": "3b0d5c1e-2f7a-4c4e-9d1b-6a8f2e7c9b10" }
headers = {
    "X-Api-Key": "<x-api-key>",
    "Content-Type": "application/json"
}

response = requests.patch(url, json=payload, headers=headers)

print(response.text)

{
  "theme": {
    "id": "3b0d5c1e-2f7a-4c4e-9d1b-6a8f2e7c9b10",
    "name": "Brand",
    "is_system_theme": false,
    "properties": [
      {
        "key": "background_color",
        "kind": "color",
        "value": "#ffffff"
      }
    ],
    "has_background_image": false
  }
}

{
  "error": "param is missing or the value is empty: theme_id"
}

{
  "error": "NeetoThemesEngine::Theme does not exist."
}