Use of `theme.json` in desktop client (and possibly other clients)

[`theme.json`](https://ocis.owncloud.com/themes/owncloud/theme.json) returns a list of `logo`s which the desktop client should display for branding purposes. Unfortunately, at the moment, the default ownCloud icon is the same for both `default` and `default-dark`: a white-filled version of the ownCloud logo.

The desktop client supports multiple themes, too. By default, we use the light theme. With the given logo, we would need to display a white logo on a white background. This doesn't work for obvious reasons.

Is it possible to provide a suitable logo on the `theme.json` endpoint? I'd expect some dark color logo for the `default` (i.e., light) theme. If the web interface requires the white logo for the light theme, too, we could provide a dedicated URL for client use. If my assumption that `default` means `light` is wrong, please correct me.

Regarding branded instances, I expect we'll have a single logo that isn't purely white but typically represents the organization that operates the instance. Here a single logo may work for both themes.