Many apps are providing night/dark mode now as it is more comforting to the eyes and improves visibility so I wanted to make a simple react app and implement the night mode. Below here is shown what I did to achieve it.

First I created a button to toggle on/off night mode.

   <button
      className="theme-btn"
      onClick={() => {}}
   />

and styled it like this

.theme-btn {
  border: none;
  width: 30px;
  height: 30px;
  border-radius: 15px;
  border-width: 1px;
  background: pink;
  border-color: pink;
  margin-right: 10px;
  cursor: pointer;
}

Then I added some text and gave styles to it, my app component looked like this

return (
    <div>
      <div className="nav">
        <button
          className="theme-btn"
          onClick={() => {}}
        />
      </div>
     <h1>
        Dark Theme
     </h1>
     <h2>
        Dark theme is so cool!
     </h2>
    </div>
 );

.nav {
  display: flex;
  flex: 1;
  flex-direction: "row";
  align-items: center;
  width: "100%";
  height: 40px;
}

After that, I initialized the state using React Hooks with its initial value as false and toggled its value between true and false upon the dark mode button click.

 const [isDarkTheme, setIsSetDarkTheme] = useState(false);

onClick method looks like this

   onClick={() => {
      setIsSetDarkTheme(!isDarkTheme);
   }}

Now I just applied conditional styling to text like this

   style = {isDarkTheme ? { color: "white" } : { color: "black" }}

I also initialized another state that will show the text of the button according to the mode selected.
Whole component looked like this

export default function App() {
  const [isDarkTheme, setIsSetDarkTheme] = useState(false);
  const [themeText, setThemeText] = useState("Go Dark");
  return (
    <div
      style={
        isDarkTheme
          ? { backgroundColor: "black" }
          : { backgroundColor: "white" }
      }
    >
      <div className="nav">
        <button
          className="theme-btn"
          onClick={() => {
            isDarkTheme ? setThemeText("Go Dark") : setThemeText("Go Light");
            setIsSetDarkTheme(!isDarkTheme);
          }}
        />
        <p style={isDarkTheme ? { color: "white" } : { color: "black" }}>
          {themeText}
        </p>
      </div>
      <h1 style={isDarkTheme ? { color: "white" } : { color: "black" }}>
        Dark Theme
      </h1>
      <h2 style={isDarkTheme ? { color: "white" } : { color: "black" }}>
        Dark theme is so cool!
      </h2>
    </div>
  );
}

You can keep isDarkTheme state in redux to use it in multiple components across the app, you can also create dark-theme and light-theme styling classes and use it across the app.

Try running the demo of Night Mode here

Implementing Night Mode in React Hooks