createThemeSubMenu() {
    const themes = [{
      label: 'Default',
      theme: 'default'
    }, {
      label: 'Elegant',
      theme: 'elegant'
    }, {
      label: 'Mdn-Like',
      theme: 'mdn-like'
    }, {
      label: 'Neat',
      theme: 'neat'
    }, {
      label: 'Neo',
      theme: 'neo'
    }, {
      label: 'Seti',
      theme: 'seti'
    }, {
      label: 'Zenburn',
      theme: 'zenburn'
    }]

    // give properties
    const selectedTheme = this.settings.get('theme')
    themes.forEach((i) => {
      const t = i
      t.type = 'radio'
      t.checked = (selectedTheme === t.theme)
      t.click = (menuItem) => {
        this.themeClicked(menuItem)
      }
    })
    return themes
  }
Create the theme sub menu. (Later versions can create this programmatically)