In one of my recent projects, I needed to add support for dark mode to a React.js Single Page Application (SPA). Since we were using SCSS modules to style our elements, let's explore how to implement dark mode in a React.js project with SCSS modules.

Switching Color Schemes

With widespread browser support for CSS variables, I don't see any alternative to using the body class + CSS variables approach. This means that when a user enables dark mode in your application, a .dark class is added to the body tag, and the variables are overridden based on the presence or absence of this class.

CSS Variables

If you're not already familiar, CSS variables (custom properties) are entities in CSS that allow you to store values and then use them in your styles. For example:

body {
  --text-color: #ccc; // define a CSS variable
}

h1 {
  color: var(--text-color); // get a variable
}

p {
  color: var(--text-color); // get a variable
}

Variable names should always start with a double hyphen (--), and you access a variable by using the var() function.

The var() function will attempt to find the --text-color variable within its scope or from its parents. In our case, that's the body. However, you can redefine this variable so that, for example, h1 and p elements inside sections have a different color:

section {
  --text-color: #ddd
}

In our case, this trick will help override variables for dark mode.

Real case

First, declare all the variables your designers use in layouts on the :root pseudo-class and add them to a global.scss file in your project:

:root {
  --black: #000;
  --gray: #ccc;
  --white: #fff;
  --blue: #0085f2;
}

In this case, I suggest not tying variable names to the entities where you plan to use them. For instance, don't name them like --text-primary-color: #ccc, because we're going to define this at the level of React component styles. For example, you have a component:

import classNames from "classnames";

import styles from "./Text.module.scss";

interface ITextProps {
  type?: "primary" | "secondary";
  children?: React.ReactNode;
}

export const Text: React.FC<ITextProps> = ({ type = "primary", children }) => {
  return (
    <p className={classNames(styles.root, styles[`${type}Type`])}>{children}</p>
  );
};

As you can see, this is a simple React Component that can have one of two types, which we intend to handle in the style file.

The style file (Text.module.scss) for this component will look like this:

.primaryType {
  --text-color: var(--blue);

  color: var(--text-color);
}

.secondaryType {
  --text-color: var(--black);

  color: var(--text-color);
}

Here, for each text type, I've defined my own variable whose value is taken from the :root pseudo-class.

Now, to enable dark mode for text, we need to use the body.dark class. We can do this as follows:

.primaryType {
  --text-color: var(--blue);

  color: var(--text-color);
}

.secondaryType {
  --text-color: var(--black);

  color: var(--text-color);
}

:global(.dark) {
  .primaryType {
    --text-color: var(--gray);
  }

  .secondaryType {
    --text-color: var(--white);
  }
}

:global(.dark) allows us to use global SCSS modules classes. Here, we simply override variable values, which, due to the nesting within the .dark class, will take higher priority than the ones declared above.

Since we are using SCSS, we can create a mixin based on this approach. Let's also add a media query to apply dark mode based on the user's operating system settings.

Here's what the mixin will look like:

SCSS Mixin

@mixin dark-mode {
  @media (prefers-color-scheme: dark) {
    @content;
  }

  :global(.dark) {
    @content;
  }
}

And here's how you can use this mixin:

@import "../styles/mixins";

.primaryType {
  --text-color: var(--blue);

  color: var(--text-color);
}

.secondaryType {
  --text-color: var(--black);

  color: var(--text-color);
}

@include dark-mode {
  .primaryType {
    --text-color: var(--gray);
  }

  .secondaryType {
    --text-color: var(--white);
  }
}

This way, dark mode styles for your components will be isolated and conveniently located at the end of the file, making it easy to navigate through them. 🌙