Summary of changes

• Adds a themes section to the tess.yml config for declaring themes & their colours (for icon generation)

Motivation and context

Previously the list of themes was hard coded in various places.

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.