[juno] Ensure that theme colors are used in the Juno apps

[ArtieReus]

Ensure that theme colors are used in the Juno apps. Avoid using fixed colors like "juno-grey-..." and instead, utilize theme colors that include the term "theme" in their name.

https://ui-team.global.cloud.sap/docs/projects/juno/juno-ui-components/colors/

Apps to check:

• assets-overview
• example-app
• greenhouse
• supernova
• template
• volta
• doop (https://github.wdf.sap.corp/cc/greenhouse-extensions/tree/main/doop/ui)