Add custom style

[TatuLund]

In the current app starter there is read me file in the folder where custom style html file should be. It would be better to have simple style html file there and use it in main view.

Idea behind of this feature request is, that it is really commonly asked question and developers to struggle to get it right (i.e. paths, correct annotation, etc.)

[Legioth]

I think we originally had something like that in the starter, along with a PolymerTemplate class and an the corresponding .html file. We removed those at some point because we wanted this starter to be as simple as possible. We instead introduced https://github.com/vaadin/beverage-starter-flow as a starter that uses a bunch of different core features.

[samie]

We already made fix to the default spring starter in vaadin/skeleton-starter-flow-spring#126 adding 3 different examples of styling. Same should go here.

[anasmi]

Changes were merged in master branch a couple of days ago #274

Closing this ticket due to it