v3.1.0

• Upgrade to marked@14
• Add theme presets

<script type="module">
  import ZeroMd, { STYLES } from 'https://cdn.jsdelivr.net/npm/zero-md@3'

  customElements.define('zero-md', class extends ZeroMd {
    async load() {
      await super.load()
      this.template = STYLES.preset('light') // or STYLES.preset('dark')
    }
  })
</script>