docs: update themes configuration settings

Description

This MR updates the documentation of theme configuration for Grove-based instances.

Supporting information

https://tasks.opencraft.com/browse/SE-5998

Testing instructions

Steps to test the changes:

1. Proofread

Dependencies

N/A

Screenshots

N/A

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated

Additional context

N/A