Add ability to add themes without redeployment

[jorgemoralespou]

Is your feature request related to a problem? Please describe.

Currently a theme needs to be pre-configured in order to be used by the session manager and training portals, as they are defined in educates config.

Describe the solution you'd like

It would be nice to have a mechanism that allowed to deploy and reference a new theme without it needing to be pre-defined in the educates config.

Describe alternatives you've considered

Create an empty theme secret and reference it by default, so that training portals uses defaults unless you as overrides in the secret. This would work for one theme.

Additional information

No response