Styles

Use the static styles getter and return your stylesheet as a string:

class MyCoolElement extends BulbaElement(Renderer) {
  static get styles() {
    return `
      :host {
        display: block;
      }

      .cool-text {
        font-family: Comic Sans MS;
      }
    `
  }

  // ...
}

A template is used here but even a plain string with basic styles will do fine.

Worth noting is that Bulba has no opinion on how you write your styles. As long as the end result is plain ol' CSS, you're good to go.