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Lightning Web Components use standard CSS for styling, with built-in Shadow DOM encapsulation that scopes styles to each component.

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CSS File Structure

Each component can have its own CSS file that shares the component’s name: 
lwc/
  myComponent/
    myComponent.js
    myComponent.html
    myComponent.css    // Component-specific styles

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Shadow DOM Scoping

Styles defined in a component’s CSS file are automatically scoped to that component using Shadow DOM. This means:
  • Styles don’t leak out to parent components
  • External styles don’t affect the component (except inherited properties)
  • Each component has its own isolated styling context
Example: lwc/contactTile/contactTile.css:1 
img {
    width: 60px;
    height: 60px;
    border-radius: 50%;
}
This img selector only affects images inside the contactTile component, not images elsewhere in the application.
Shadow DOM scoping eliminates CSS conflicts and makes components truly modular and reusable.

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The :host Selector

Use the :host pseudo-class to style the component’s host element itself: 
:host {
    display: block;
    padding: 1rem;
}
This is commonly used to:
  • Set the display mode of the component
  • Define CSS custom properties for child components
  • Apply default spacing or layout

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SLDS Design Tokens

Lightning Web Components have access to Salesforce Lightning Design System (SLDS) design tokens through CSS custom properties: Example: lwc/eventSimple/eventSimple.css:2 
fieldset {
    border: solid var(--slds-g-sizing-border-1)
        var(--slds-g-color-neutral-base-80);
    border-radius: var(--slds-g-radius-border-2);
}
Common SLDS token categories:
  • --slds-g-color-* - Color tokens
  • --slds-g-sizing-* - Sizing and spacing tokens
  • --slds-g-font-* - Typography tokens
  • --slds-g-radius-* - Border radius tokens
Use SLDS design tokens instead of hard-coded values to ensure your components match the Salesforce design system and respond to theme changes.

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Multiple Stylesheets

You can import additional CSS files using the static stylesheets property: Example: lwc/stylesheets/stylesheets.js:1 
import { LightningElement } from 'lwc';
import textStyles from './text-styles.css';

export default class Stylesheets extends LightningElement {
    static stylesheets = [textStyles];
}
Main stylesheet: lwc/stylesheets/stylesheets.css:1 
.card-body {
    background-color: yellow;
}
Additional stylesheet: lwc/stylesheets/text-styles.css:1 
.fancy-text {
    color: #d54d7b;
    font-family: 'Great Vibes', cursive;
    font-weight: var(--slds-g-font-weight-4, 400);
    text-align: center;
    text-shadow: 0 1px 1px #fff;
}
Both stylesheets are scoped to the component and applied together.
Use multiple stylesheets to organize complex styling or share common styles between related components.

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Styling Base Components

You can style Lightning base components using class names: 
<template>
    <lightning-input 
        class="custom-input"
        label="Name"
    ></lightning-input>
</template>

.custom-input {
    margin-bottom: 1rem;
}

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Best Practices

  • Use SLDS utility classes in templates: class="slds-var-m-around_medium"
  • Leverage SLDS design tokens for consistent styling
  • Keep component styles focused and minimal
  • Use :host for component-level layout
  • Avoid !important - Shadow DOM scoping makes it unnecessary
  • Use semantic class names that describe purpose, not appearance

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