Theming

A theme is a coherent style applied to UI components. Themes make it easier to customize your app's design for a specific brand or app state (for example, 'light mode' and 'dark mode').

To create a theming system with Bit and Angular, we will follow these steps:

1. create a base theme component
2. create a custom theme component
3. lazy load themes
4. load your themes in your compositions

If you are not interested in the details, you can check out the full version of the base theme component.

A theme requires a base theme component to apply the same style values on multiple UI components. For that, we will use the scss template provided by the HTML environment. Run the following command to create a new base theme component:

$bit create scss theme/my-base-theme --aspect teambit.html/html-env

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Usually, designers create design tokens with specialized design software, such as Figma, and export them to different formats, depending on the needs. For simplicity, we will start with a scss file in our base component.

Design tokens are usually maintained in a JSON format due to its broad support in many programming languages.

We will store the design tokens as scss variables in your base theme component's file named my-base-theme.module.scss.

$myFontSize: 16px;
$myFontFamily: "Roboto, sans-serif";
$myBorderRadius: 3px;
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There are multiple ways to export and use your design tokens:

• You can import the scss file and use the scss variables directly.
• You can create a mixin that will output the tokens as css variables.
• You can export the tokens to js and use them in your code.

We will use the second option as it is similar to what Angular Material uses, but you can choose the one that fits your needs.

Here is your base theme component with the design tokens:

// Design tokens
$myFontSize: 16px;
$myFontFamily: "Roboto, sans-serif";
$myBorderRadius: 3px;

// Mixin to output design tokens as CSS variables
@mixin myBaseTheme() {
  --myFontSize: $myFontSize;
  --myFontFamily: $myFontFamily;
  --myBorderRadius: $myBorderRadius;
}

:export {
  myFontSize: $myFontSize;
  myFontFamily: $myFontFamily;
  myBorderRadius: $myBorderRadius;
}
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Since we chose to use css variables, the theme should be global and applied to your application's body element to ensure that all your components get access to the variables. In your application's root styles.scss file, import your base theme and include the mixin to apply the theme to the body element. You can now use the base theme variables in your application:

@use "@my-org/my-scope.theme.my-base-theme/my-base-theme.module" as bt;

body {
  @include bt.myBaseTheme();
  font-size: var(--myFontSize, 16px);
}
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To reuse this base theme, check out the full version on Bit Cloud.

Create a new component to set a dark theme for your apps:

$bit create scss theme/my-dark-theme

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Similarly to the base theme, we can create the new design tokens for the dark theme and export them as css variables:

// Design tokens
$myPrimaryColor: #4571c4;
$mySecondaryColor: #c42020cc;
$myTextColor: #bae0ff;
$myBgColor: #303030;

// Mixin to output design tokens as CSS variables
@mixin myDarkTheme() {
  --myPrimaryColor: $myPrimaryColor;
  --mySecondaryColor: $mySecondaryColor;
  --myTextColor: $myTextColor;
  --myBgColor: $myBgColor;
}

:export {
  myPrimaryColor: $myPrimaryColor;
  mySecondaryColor: $mySecondaryColor;
  myTextColor: $myTextColor;
  myBgColor: $myBgColor;
}
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You can apply the mixin of your custom dark theme directly to your application's body element like we just did for the base theme. But since the dark theme is a custom theme, it is better to apply it to a specific class:

@use "@my-org/my-scope.theme.my-dark-theme/my-dark-theme.module" as dt;

body {
  &.dark-theme {
    @include dt.myDarkTheme();
  }

  background: var(--myBgColor, 16px);
}
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To reuse this dark theme, check out the full version on Bit Cloud.

Once you have multiple custom themes, you can lazy load them to reduce the initial bundle size of your application. The first step is to create a new entry point for each theme. Create a new file for each theme in your' src' folder, for example, themes/light-theme.scss and themes/dark-theme.scss. In each file, import the custom theme and apply the custom theme to the body element:

@use "@my-org/my-scope.theme.my-dark-theme/my-dark-theme.module" as dt;

body {
  @include dt.setDarkTheme();
}
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Angular provides options to generate multiple external style files that won't be bundled with your application. In your application, open the root config file of your application (my-angular-app.ng-app.ts) and change the styles option to the following array:

styles: [
  './src/styles.scss',
  {
    input: './src/themes/light-theme.scss',
    inject: false,
    bundleName: 'light-theme',
  },
  {
    input: './src/themes/dark-theme.scss',
    inject: false,
    bundleName: 'dark-theme',
  },
]
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Usually, a theme switcher component will load the theme based on the user's preference. For simplicity, we will use a button to load the dark theme. In your app.component.html file, add a button to load the dark theme:

<button (click)="loadDarkTheme()">Load dark theme</button>
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In your app.component.ts file, add the loadDarkTheme method to load the dark theme:

import { Component, Inject, Renderer2 } from '@angular/core';
import { DOCUMENT } from '@angular/common';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss'],
})
export class AppComponent {
  constructor(private renderer: Renderer2, @Inject(DOCUMENT) private document: Document) {}

  loadDarkTheme() {
    const link = this.renderer.createElement('link');
    link.rel = 'stylesheet';
    link.href = './dark-theme.css';
    this.renderer.appendChild(this.document.head, link);
  }
}
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You can now load the dark theme by clicking the button.

The simplest way to load your themes for all your compositions is to configure your custom environment mount file with a wrapper component. Setting ViewEncapsulation.None on the wrapper component removes the scope of your wrapper styles and lets your composition inherit the wrapper's styles. You can also use :host ::ng-deep in your theme styles to affect all descendant elements.

The following example wraps every composition with your base theme:

// @filename: my-angular-env.bit-env.ts
import { AngularV18Env } from '@bitdev/angular.envs.angular-v18-env';
import { Preview } from '@teambit/preview';
import { EnvHandler } from '@teambit/envs';
import { AngularPreview } from '@bitdev/angular.dev-services.preview.preview';

export class MyAngularEnv extends AngularV18Env {
  preview(): EnvHandler<Preview> {
    return AngularPreview.from({
      mounterPath: require.resolve('./preview/mounter')
    });
  }
}

export default new MyAngularEnv();
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