Theming with CSS variables.

/* Define your variables to the root. */
:root {
  --black: #181818;
  --white: #f5f7f7;
  --fade:  150ms;
}

/*  
The variables of the tag with the data-theme="light" attribute.
We will then apply this property to the HTML tag with javascript,
but you can define it manually.
*/
[data-theme="light"]{
  --color-text: var(--black);
  --color-background: var(--white);
}

/* Change variables for dark theme. */
[data-theme="dark"]{
  --color-text: var(--white);
  --color-background: var(--black);
}

/* And apply styles to document root element. */
html {
  color: var(--color-text);
  background-color: var(--color-background);

  /* for smooth color transitions. */
  transition: var(--fade) color, var(--fade) background-color;
}

// Call theme value from browsers local storage.
var theme = localStorage.getItem("theme");
// Root element of the document.
const _root = document.documentElement;

// Check if local storage has no theme value, define the initial value.
!theme && localStorage.setItem("theme", "light");

// Update theme value.
theme = localStorage.getItem("theme");

// Apply theme value to document root element.
_root.setAttribute("data-theme", theme);

// Function for change theme.
// You can define this function to the a button or call this any way.
function changeTheme() {
  theme === "light"
    ? localStorage.setItem("theme", "dark")
    : localStorage.setItem("theme", "light");

  // Update theme value
  theme = localStorage.getItem("theme");

  // Apply theme to document root element
  _root.setAttribute("data-theme", theme);
}