Global CSS in NextJS

Introduction

Global CSS lets you apply styles that affect your whole website. It helps keep your design consistent everywhere.

You want the same font and colors on every page.

You need to style HTML elements like body, h1, or p globally.

You want to add a background color or reset default browser styles.

You want to include third-party CSS libraries like Bootstrap.

You want to define CSS variables for your entire app.

Syntax

NextJS

/* In your Next.js project, create a CSS file like styles/globals.css */

/* styles/globals.css */
body {
  margin: 0;
  font-family: Arial, sans-serif;
  background-color: #f0f0f0;
}

/* Then import it in your app entry file */

// app/layout.js (Next.js 13+ App Router)
import './globals.css';

export default function RootLayout({ children }) {
  return (
    <html lang="en">
      <body>{children}</body>
    </html>
  );
}

Global CSS files must be imported only once, usually in the root layout or _app.js.

Next.js requires global CSS imports to be in special files like app/layout.js or pages/_app.js.

Examples

This example sets a white background and black text for the whole site.

NextJS

/* styles/globals.css */
body {
  background-color: white;
  color: black;
}

// app/layout.js
import './globals.css';

This example styles all <h1> headings globally with a bigger font and dark blue color.

NextJS

/* styles/globals.css */
h1 {
  font-size: 2rem;
  color: darkblue;
}

// app/layout.js
import './globals.css';

Sample Program

This Next.js app uses a global CSS file to style the body and h1 elements. The background is a light teal color, text is dark teal, and the heading is centered with margin.

NextJS

/* styles/globals.css */
body {
  margin: 0;
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  background-color: #e0f7fa;
  color: #006064;
}

h1 {
  text-align: center;
  margin-top: 2rem;
}

// app/layout.js
import './globals.css';

export default function RootLayout({ children }) {
  return (
    <html lang="en">
      <body>
        <h1>Welcome to My Next.js Site</h1>
        {children}
      </body>
    </html>
  );
}

Important Notes

Global CSS affects all components and pages, so use it for shared styles only.

For component-specific styles, use CSS Modules or styled components instead.

Remember to keep your global CSS file organized to avoid conflicts.

Summary

Global CSS applies styles across your entire Next.js app.

Import your global CSS file once in app/layout.js or pages/_app.js.

Use global CSS for fonts, colors, and base element styles.

Practice

1. In Next.js, where should you import your global CSS file to apply styles across the entire app?

easy

A. In app/layout.js or pages/_app.js

B. In every individual component file

C. Inside public folder

D. In next.config.js

Solution

1. Step 1: Understand global CSS import location

   Global CSS must be imported once at the top-level layout or app file to apply styles app-wide.

2. Step 2: Identify correct files for global CSS import

   Next.js uses app/layout.js in the new app directory or pages/_app.js in the pages directory for this purpose.

3. Final Answer:

   In app/layout.js or pages/_app.js -> Option A

4. Quick Check:

   Global CSS import location = A [OK]

Hint: Global CSS imports go only in app/layout.js or pages/_app.js [OK]

Common Mistakes:

• Importing global CSS in every component causes errors
• Placing CSS files in public folder expecting auto-import
• Trying to import CSS in next.config.js

2. Which of the following is the correct syntax to import a global CSS file named styles.css in app/layout.js?

easy

A. import styles from './styles.css';

B. import './styles.css';

C. require('./styles.css');

D. link rel='stylesheet' href='./styles.css';

Solution

1. Step 1: Recall global CSS import syntax in Next.js

   Global CSS files are imported without assigning to a variable, just using import './filename.css';.

2. Step 2: Evaluate each option

   import styles from './styles.css'; tries to import as a module which is for CSS modules, not global CSS. require('./styles.css'); uses CommonJS require which is not standard in Next.js ES modules. link rel='stylesheet' href='./styles.css'; is HTML syntax, not JavaScript import.

3. Final Answer:

   import './styles.css'; -> Option B

4. Quick Check:

   Global CSS import syntax = D [OK]

Hint: Use plain import './filename.css'; for global CSS [OK]

Common Mistakes:

• Using import with variable for global CSS
• Using require() instead of import
• Trying to add <link> tags inside JS files

3. Given this app/layout.js snippet:

import './globals.css';
export default function RootLayout({ children }) {
  return <html><body>{children}</body></html>;
}

If globals.css sets body { background-color: lightblue; }, what will be the background color of the page?

medium

A. White (default background)

B. Black

C. Transparent

D. Lightblue

Solution

1. Step 1: Understand global CSS effect

   Importing globals.css in app/layout.js applies styles globally, including the body background color.

2. Step 2: Check CSS rule impact

   The CSS rule body { background-color: lightblue; } sets the page background color to lightblue.

3. Final Answer:

   Lightblue -> Option D

4. Quick Check:

   Global CSS body background = B [OK]

Hint: Global CSS affects body styles if imported in layout [OK]

Common Mistakes:

• Thinking global CSS won't apply without explicit class
• Assuming default white background overrides CSS
• Confusing local CSS modules with global CSS

4. You added import './globals.css'; inside a React component file instead of app/layout.js. What problem will you most likely face?

medium

A. Next.js build error about global CSS import location

B. CSS styles will apply only to that component

C. No styles will apply anywhere

D. Styles will load twice causing duplication

Solution

1. Step 1: Recall Next.js global CSS import rules

   Next.js requires global CSS to be imported only in app/layout.js or pages/_app.js. Importing elsewhere causes errors.

2. Step 2: Identify error behavior

   Importing global CSS inside a component triggers a build error stating global CSS cannot be imported from non-top-level files.

3. Final Answer:

   Next.js build error about global CSS import location -> Option A

4. Quick Check:

   Global CSS import location error = A [OK]

Hint: Global CSS import outside layout causes build error [OK]

Common Mistakes:

• Importing global CSS in components expecting scoped styles
• Ignoring build errors and expecting styles to work
• Confusing global CSS with CSS modules

5. You want to set a global font and base color for your Next.js app. Which approach correctly applies these styles globally and ensures they affect all pages?

hard

A. Use CSS modules in each component to set font and color

B. Add inline styles to each page component separately

C. Create globals.css with font and color styles, then import it once in app/layout.js

D. Add a <style> tag with font and color in pages/index.js only

Solution

1. Step 1: Understand global styling needs

   Global fonts and colors should be set once to apply app-wide, not repeated per page or component.

2. Step 2: Choose correct global CSS usage

   Creating a globals.css file with base styles and importing it once in app/layout.js ensures consistent global styles.

3. Final Answer:

   Create globals.css and import it once in app/layout.js -> Option C

4. Quick Check:

   Global base styles = C [OK]

Hint: Put base styles in globals.css and import once in layout [OK]

Common Mistakes:

• Using inline styles repeatedly instead of global CSS
• Relying on CSS modules for global styles
• Adding styles only on one page expecting global effect