Assets are automatically compiled when running mvn package. If you change assets, and want them updated in your jar, you must either rm -rf src/main/resources/assets/build or mvn clean.
To modify assets and have changes reflected on the fly, you will need to start Chronos with a specially configured YAML file. The configuration is quite simple and is documented at dropwizard-configurable-assets-bundle.
There are also two included example YAML files that make use of this configuration: