retrieve:
Return the details about the given Theme id.

list:
List all Theme objects.

GET /api/v1/themes/3/
HTTP 200 OK
Allow: GET, HEAD, OPTIONS
Content-Type: application/json
Vary: Accept

{
    "id": 3,
    "fr_label": "Informatique",
    "en_label": "Computer Science",
    "de_label": "Informatik"
}