Create a demo for the application theme with global styles

[mshabarov]

Create a demo for the feature in vaadin/application-theme-demo which showcases the in-project app theme usage with some global css applied to the Vaadin crm tutorial end result. https://github.com/vaadin-learning-center/crm-tutorial

It will be initially just a module for “in-project demo” and later on for reusable case. It should showcase

• global styles
• component specific theming with e.g. applying borders to all text-fields
• background image used via css
• background image used via Java code
• using a custom font from a npm package ?
• using font-awesome icons from an npm package

Part of #9246

[taefi]

background image used via Java code is waiting for #9535 and #9622

[taefi]

After #9622 is merged, the background image used via Java code is working now.