:host {
  font-family: 'Roboto', 'Arial', 'Helvetica', sans-serif;
}

polyfill-next-selector { content: ':host > *'; }
::content > * {
  min-width: 60px;
  height: 60px;
  line-height: 60px;
  text-align: center;
  padding: 0 8px;
  font-size: 14px;
  color: #999;
}

polyfill-next-selector { content: ':host > .core-selected'; }
::content > .core-selected {
  background: #5599FF;
  color: white;
}
:host {
  font-family: 'Roboto', 'Arial', 'Helvetica', sans-serif;
}

polyfill-next-selector { content: '*'; }
::content > * {
  min-width: 60px;
  height: 60px;
  line-height: 60px;
  text-align: center;
  padding: 0 8px;
  font-size: 14px;
  color: #999;
}

polyfill-next-selector { content: '.core-selected'; }
::content > .core-selected {
  background: #5599FF;
  color: white;
}

Drop :host portion of selector to fix FF issues.
Drop :host portion of selector to fix FF issues.