Global CSS in NextJS - Step-by-Step Execution

Concept Flow - Global CSS

Create global CSS file

↓

Import global CSS in _app.js

↓

Next.js applies styles globally

↓

All pages/components share styles

↓

Styles cascade and override as usual

Global CSS files are created and imported once in the main app file, then Next.js applies these styles across all pages and components.

Execution Sample

NextJS

/* styles/globals.css */
body { margin: 0; font-family: Arial; }

// pages/_app.js
import '../styles/globals.css'

export default function App({ Component, pageProps }) {
  return <Component {...pageProps} />
}

This code imports a global CSS file in the main app component so styles apply to the whole Next.js app.

Execution Table

Step	Action	File	Effect	Result
1	Create global CSS file with styles	styles/globals.css	Defines body margin and font	CSS file ready
2	Import global CSS in _app.js	pages/_app.js	Next.js loads CSS globally	Styles available app-wide
3	Render App component	pages/_app.js	Apply global styles to body	All pages use Arial font, no margin
4	Navigate to any page	any page	Global styles persist	Consistent look across pages
5	Override styles in component CSS	component CSS	Overrides global styles locally	Local styles take precedence
6	Exit	N/A	No more steps	Global CSS applied successfully

💡 All steps complete; global CSS loaded and applied across the app

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
globalCSS	undefined	imported	applied	applied globally
bodyStyle	default browser	overridden by global CSS	applied on render	Arial font, margin 0

Key Moments - 2 Insights

Why do we import the global CSS only in _app.js?

Because _app.js wraps all pages, importing global CSS here ensures styles apply everywhere once, as shown in execution_table step 2.

What happens if a component has its own CSS that conflicts with global CSS?

Component CSS overrides global CSS locally due to CSS cascade rules, as seen in execution_table step 5.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the global CSS first applied to the app?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Check the 'Effect' column for when styles are applied on render.

According to variable_tracker, what is the state of bodyStyle after step 3?

A default browser style

B overridden by global CSS

C applied on render

D undefined

💡 Hint

Look at the 'After Step 3' column for bodyStyle.

If you remove the import from _app.js, what would happen to the global CSS?

A It would still apply globally

B It would not apply anywhere

C It would apply only to _app.js

D It would apply only to pages but not components

💡 Hint

Refer to key_moments about why import in _app.js is necessary.

Concept Snapshot

Global CSS in Next.js:
- Create a CSS file (e.g., styles/globals.css)
- Import it once in pages/_app.js
- Styles apply globally to all pages/components
- Local component CSS can override global styles
- Ensures consistent styling across the app

Full Transcript

Global CSS in Next.js means creating a CSS file with styles you want everywhere. You import this file once in the special _app.js file. This file wraps all pages, so importing here makes the styles global. When the app renders, these styles apply to the whole app, like setting the font or margin on the body. If a component has its own CSS, it can override these global styles locally. This way, you keep a consistent look but still customize parts. Without importing the global CSS in _app.js, the styles won't apply anywhere.

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