Dark Theme on LitElement App
Tags: webdev, css
Published: 2020-09-07 15:58:05

Building a component or complex app on top of LitElement means keep in mind the use of CustomElement, Template and Shadow DOM in a javascript coded element. In this easy post i'll show a simple web app shell based on MyApp LitElement component in charge of sync the dark property, used to style in light and dark mode the app element, the app's childs elements and the upper body parent.

```javascript
export class MyApp extends LitElement {
  
  static get properties() {
    return {
      dark: {
        type: Boolean,
        reflect: true
      }
    }
  }

  constructor() {
    super()
    this.dark = false
  }
static get styles() {

    return css`

      /* handle the light / dark mode */ 
      :host:not([dark]) {
        --bk-color: #fff;
        --bk-second-color: #e0e0e0e;
      }
      :host([dark]) {
        --bk-color: #666;
        --bk-second-color: #004c8c;
      }

      :host {
        display: block;
        margin: 2em;
        padding: 2em;
        background-color: var(--bk-color, #fff);
      }

      #container {
        display: block;
        margin: 2em;
        padding: 1em;
        text-align: center;
        background-color: var(--bk-second-color, #e0e0e0);
      }
    `
  }

  _checked(event) {
    this.dark = event.target.checked
  }

  render() {
    return html`
      <div id="container">
        <h1>${this.dark ? 'DARK' : 'LIGHT'} Theme</h1>
        <hr>
        <input type="checkbox" id="dark_label" name="dark" @click="${this._checked}">
        <label for="dark"> Dark Theme</label><br>

        <child-component ?dark="${this.dark}"></child-component>
      </div>
    `
  }
``` 

This code reflect the dark property to the app custom element my-app, this attribute is handled in the index.html in case the parent body needs to use the dark property to set some CSS custom vars outside the my-app application shell:
```html
  <style>

    html {
      /* light and dark custom vars */
      --bk-color: #fff;
      --color: #222;
      
      --bk-color-dark: #222;
      --color-dark: whitesmoke;
    }

    body:not([dark]) {
      /* custom var and fallback value */ 
      background-color: var(--bk-color, #fff);
      color: var(--color, #222)
    }
    body[dark] {
      /* custom var and fallback value */ 
      background-color: var(--bk-color-dark, #121212);
      color: var(--color-dark, whitesmoke)
    }

    body  {
      margin: 0;
      font-family: Roboto, Helvetica, Arial, sans-serif;
      padding: 2em;
      text-align: center;
    }

  </style>
  
</head>

<body>
  
  <!-- LitElement with dark attribute -->
  <my-app></my-app>

  <!-- Import Js Module -->
  <script type="module" src="my-app.js"></script>

  <script>
    // propagate the attribute of app to body 
    const body = document.querySelector('body')
    const app = document.querySelector('my-app')
    const config = { attributes: true, childList: false, subtree: false };
    const callback = (mutationsList, observer) => {
      for(let mutation of mutationsList) {
        if (mutation.type === 'attributes' && mutation.attributeName === 'dark') {
            // console.log(`@DARK >> ${app.hasAttribute('dark')}`)
            app.hasAttribute('dark') ? 
              body.setAttribute('dark', '') :
              body.removeAttribute('dark')
        }
      }
    }

    const observer = new MutationObserver(callback)
    observer.observe(app, config)
  </script>

</body>
```

The dark property go down in the DOM tree to be shared inside all the child that needs the dark property to trigger some UI difference between light and dark theme:
```javascript
export class ChildComponent extends LitElement {

  static get properties() {
    return {
      dark: {
        type: Boolean,
        reflect: true
      }
    }
  }

  static get styles() {
    return css`

    /* handle the light / dark mode */ 
    :host:not([dark]) {
      --bk-color: #eee;
    }
    :host([dark]) {
      --bk-color: #0069c0;
    }

    :host {
      display: block;
    }
  
    div {
      width: 50%;
      height: 150px;
      margin-right: auto;
      margin-left: auto;
      margin-top: 1em;
      margin-bottom: 1em;
      padding: 2em;
      background-color: var(--bk-color, #fff);
    }
    `
  }

  render() {
    return html`
      <div>
        <h2>Child Component</h2>
      </div>
    `
  }
}

customElements.define('child-component', ChildComponent)
```

Here the github repo! (Thanks to spot the bugs!! ) 
https://github.com/CICCIOSGAMINO/dark-theme