Struggling with CSS sharing? With Summer ’20 it’s now easier to share CSS styles. To share a CSS file among different components, create a CSS-only module. A CSS-only module contains a CSS file and a metadata file.

Import the module into the CSS file of any Lightning web component you want to style. You can import style rules from multiple CSS modules. The imported style rules are applied to the template just like non-imported style rules. Lightning Web Components supports the CSS cascade algorithm.

Steps :

• Create a Lightning web component that contains only a single CSS file

This component is your CSS module.

cssLibrary
   ├── cssLibrary.css
   └──cssLibrary.js-meta.xml

• Import the CSS module into a Lightning Web Component

/* Syntax */
@import 'namespace/moduleName';

/* Example */
@import 'example/cssLibrary';

Note :

1. You can import as many modules as you need.
2. You can also use the SLDS design token for LWC components.

Important! 

1. The module should only have a CSS file & Metadata file in it, otherwise it will not work.
2. Just like with any Lightning web component, the folder name and the filename must be identical.
3. This feature is available in Summer ’20 Release

Let’s do it with Example

Step 1) Create Shared CSS Library

• Create a Lightning web component

• • Lightning web component will come with cssLibrary.html, cssLibrary.js & cssLibrary.js-meta.xml files, add the cssLibrary.css in the component.
  • Delete cssLibrary.html & cssLibrary.js file from component.
  • Now your component contains only cssLibrary.css & cssLibrary.js-meta.xml files. See screenshot above.

• Write the common CSS styles in the cssLibrary.css.

/* cssLibrary.css */
h1 {
    color: darkorchid;
    font-size: xx-large;
}
p {
    color: yellowgreen;
    font-size: larger;
}

Step 2) How to use shared Css in Other LWC

Create new Lightning web component (cssSharingComponent) to use the above shared CSS styles

Below are the files contents

cssSharingComponent.html

<!-- cssSharingComponent.html -->
<template>
    <lightning-card title="CSS Sharing demo" class="slds-m-around_x-large">
        <h1>Demo of CSS Sharing</h1>
        <p>Simple demo of CSS sharing feature.</p>
    </lightning-card>
</template>

cssSharingComponent.css

/* cssSharingComponent.css */
@import 'c/cssLibrary'; 

cssSharingComponent.js

// cssSharingComponent.js
import { LightningElement } from 'lwc';
	
export default class CssSharingComponent extends LightningElement {}

cssSharingComponent.js-meta.xml

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>49.0</apiVersion>
    <isExposed>true</isExposed>
</LightningComponentBundle>

As per CSS sharing we have shared all CSS styles of our cssLibrary component in cssSharingComponent.

Put your cssSharingComponent in Lightning App to test the result/output.

CssSharingDemoApp.app

<!-- CssSharingDemoApp.app -->
<aura:application extends="force:slds">
<c:cssSharingComponent />
</aura:application> 

Result/Output: In the above example, we created a CSS module called c-css-library and imported it into a component called c-css-sharing-component.

The styles in c-css-library apply to c-css-sharing-component. So the h1 & p tag will be styled as specified in the shared CSS module.

This is the result: