title: vaadin component setstylename has no effect
body: <p>The following should work. But it does not. (Css change has no effect whatsoever) What went wrong?</p>
  
  <pre><code>@Theme("mytheme")
  @Widgetset("de.datalovers.MyAppWidgetset")
  public class MyUI extends UI {
  
      @Override
      protected void init(VaadinRequest vaadinRequest) {
          verticalLayout = new VerticalLayout();
          String s = "Montag 07:30 - 12:00"
          Label label = new Label();
          label.setStyleName("mystyle");
  </code></pre>
  
  <p>mytheme.sccs:</p>
  
  <pre><code>    @import "../valo/valo.scss";
  
  @mixin mytheme {
    @include valo;
  
    // Insert your own theme rules here
  
    .mystyle{
  
      color: blue;
      background: yellow;
      font-size: 40px;
  
    }
  }
  </code></pre>
  
  <p>styles.sccs</p>
  
  <pre><code>@import "mytheme.scss";
  @import "addons.scss";
  
  // This file prefixes all rules with the theme name to avoid causing conflicts with other themes.
  // The actual styles should be defined in mytheme.scss
  
  .mytheme {
    @include addons;
    @include mytheme;
  }
  </code></pre>