How to change CSS dynamically for whole site with vite and lit?

Dear all I am developing a node app with vite and lit. I like to have the option that the user can change the style dynamically at runtime for instance to select between a dark and bright theme for the whole site.

I know that you can directly import "css" files and apply it on single lit elements. But how I can change the style dynamically for the whole site? I also noticed that an ìmport from "xxxx.css" can not contain a dynamic variable,so the following seems not to be supported?

let theme = "dark"
import "folder/" + theme + ".css";

Any suggestion how I can best achieve my goal?

Solution

This is best handled by setting all your colors in your elements as CSS Custom Properties. e.g.

class MyCustomCard extends LitElement {
  static styles = css`
    .card {
      /* Set custom property and default value */
      background-color: var(--sys-color-surface, #FFF);
      color: var(--sys-color-on-surface, #000);
      ...
    }
  `;
  ...
}

Then you can have a theming.css file at the root of your document. e.g.

<!-- index.html -->
<head>
  <link rel="stylesheet" href="theme.css">
</head>

/* theme.css */
/* light theme */
:root, body.force-light-mode {
  --sys-color-surface: #FFF;
  --sys-color-on-surface: #000;
  ...
}
/* auto dark theme */
@media (prefers-color-scheme: dark) {
  :root {
    --sys-color-surface: #000;
    --sys-color-on-surface: #FFF;
    ...
  }
}

/* user preference dark mode */
body.force-dark-mode {
  /* This can be deduplicated with an @import https://developer.mozilla.org/en-US/docs/Web/CSS/@import */
  --sys-color-surface: #000;
  --sys-color-on-surface: #FFF;
  ...
}

Then you can have a dark theme toggle that toggles the force-*-mode class on body:

class ThemeToggle extends LitElement {
  render() {
    return html`
      <div>Select Theme</div>
      <label>
        Light Mode
        <input
            type="radio"
            @change=${this.onChange('light')}
            name="theme">
       </label>
      <label>
        Follow System
        <input
            type="radio"
            @change=${this.onChange('system')}
            name="theme"
            checked>
       </label>
      <label>
        Dark Mode
        <input
            type="radio"
            @change=${this.onChange('dark')}
            name="theme">
       </label>`
  }

  onChange = (mode) => (e) => {
    if (e.target.checked) {
      switch(mode) {
        case 'light':
          document.body.classList.add('force-light-mode');
          break;
        case 'dark':
          document.body.classList.add('force-dark-mode');
          break;
      }
    } else {
      switch(mode) {
        case 'light':
          document.body.classList.remove('force-light-mode');
          break;
        case 'dark':
          document.body.classList.remove('force-dark-mode');
          break;
      }
    }
  }
}