Start with the parts already written.
Pick the AppJars modules you need. You get a Maven project — Spring Boot, Vaadin 25 — with security, navigation, properties and theme already wired for those modules. Unzip, mvn spring-boot:run, done.
No license key needed. Every module runs in free mode with record and operation limits, which is enough to evaluate it.
Modules
Project
Java
Database
Appearance
plain CSS variables
Primary color
--lumo-primary-color
Secondary color
Written to --app-secondary-color
styles.css as custom properties. Edit them later in any text editor.