Use option passing to nominate a PlantUML theme

[sturtison]

This is a possible follow on application of the Kroki's new capability to pass options to Kroki using the kroki-diagram-options-key-view mechanism.

Originally posted by @Mogztter in #827 (comment)

[sturtison]

This may be one way of addressing using Kroki to achieve a common look and feel for generated diagrams without requiring changes to the diagrams themselves, A change is made to the Antora playbook, Antora component file (antora.yml), asciidoc attributes or other system, or even simply the Kroki server itself, depending on where the standardisation is to be applied or overridden.