WordPress/wp-content/themes/twentytwentyone/assets/sass/style-dark-mode.scss

/* OS dark theme preference */
@media only screen {

	.is-dark-theme.is-dark-theme {
		--global--color-background: var(--global--color-dark-gray);
		--global--color-primary: var(--global--color-light-gray);
		--global--color-secondary: var(--global--color-light-gray);
		--button--color-text: var(--global--color-background);
		--button--color-text-hover: var(--global--color-secondary);
		--button--color-text-active: var(--global--color-secondary);
		--button--color-background: var(--global--color-secondary);
		--button--color-background-active: var(--global--color-background);
		--global--color-border: #9ea1a7;

		.site a:focus,
		.site a:focus .meta-nav {
			background: #000;
			color: #fff;
			text-decoration: none;
		}

		img {
			filter: brightness(.85) contrast(1.1);
		}
	}

	.respect-color-scheme-preference.is-dark-theme body {
		background-color: var(--global--color-background);
	}

	#dark-mode-toggler {
		cursor: pointer;
		display: flex;
		align-items: center;
		justify-content: center;
		font-size: var(--global--font-size-xs);
		padding: 0.5em;
		min-height: 44px; // A11y requirement for minimum clickable area.
		min-width: max-content;
		border: 2px solid currentColor;
		box-shadow: none;
		background: var(--button--color-text);
		color: var(--button--color-background);
		z-index: 9998;

		.no-js & {
			display: none;
		}

		&.fixed-bottom {
			position: fixed;
			bottom: 5px;
			right: 5px;
			transition: bottom 0.5s;

			&.hide:not(:focus) {
				bottom: -80px;
			}
		}

		&.relative {
			position: absolute;
			height: 44px;
			top: calc(2.4 * var(--global--spacing-vertical) - 44px);
			right: calc(50vw - var(--responsive--alignwide-width) / 2 - 0.5em);

			.admin-bar & {
				top: calc(2.4 * var(--global--spacing-vertical) - 44px + 32px);

				@media only screen and (max-width: 782px) {
					top: calc(2.4 * var(--global--spacing-vertical) - 44px + 46px);
				}

				@media only screen and (max-width: 481px) {
					top: calc(2.4 * var(--global--spacing-vertical) - 44px + 26px);
				}
			}

			~ nav {

				body:not(.primary-navigation-open) & {
					@media only screen and (max-width: 481px) {
						top: calc(44px + 44px);
					}
				}
			}
		}

		.primary-navigation-open & {
			display: none;
		}

		&:hover,
		&:focus {
			color: var(--button--color-background-active);
			border: 2px solid var(--button--color-text-active);
			background-color: var(--button--color-text-active);
		}

		.is-IE & {
			display: none;
		}
	}
}