1) If you don't specify a style, VAADIN will use the default reindeer-css classnames. If you want custom styling, either override the reindeer styles CSS-classes or add new custom classnames.
2) Not sure what you're asking, can you rephrase? The css (reindeer) and custom css are included in the HEAD of the html. Any styles you add in the explorer style css files will be available. Nothing special is required, this is just plain CSS/HTML.