I cannot use this in the vCD admin ui (as it requires .swf) – is this to be modified directly in the Clarity integration of vCD?

There are some new API calls to upload and enable the custom theme usage.

You'd first create a new theme:

POST https://<vcd api base>/cloudapi/branding/themes
Body:
{
  "name": "<custom theme name>"
}

Then issue a request to upload the generated CSS file using the transfer service:

POST https://<vcd api base>/cloudapi/branding/themes/<custom theme name>/contents
Body:
{
  "fileName": "<name-of-file>",
  "size": <size-of-css-in-bytes>
}

The POST will return a Link header that contains the upload location.  You would then do a `PUT` to the upload link with the CSS file's binary data.

Once you've done that, you should be able to do

PUT https://<vcd api base>/cloudapi/branding
Body:
{
  "portalName": "string",
  "portalColor": "string",
  "selectedTheme": {
   "name": "<custom theme name>"
  }
}

To set the branding for both the provider and tenant H5 portals.