wp-calypso/packages/help-center/src/components/back-to-top-button.scss

$animation-timing: 300ms;

.help-center__container {
	.back-to-top-button__help-center {
		position: sticky;
		bottom: 0;
		left: 0;
		width: 100%;

		display: flex;
		align-items: center;
		justify-content: center;

		background-color: var(--studio-white);
		color: var(--studio-black);

		border-radius: 0;
		border-top: 1px solid rgba(0, 0, 0, 0.1);

		visibility: hidden;
		opacity: 0;
		transition: visibility 0s linear $animation-timing, opacity $animation-timing;

		&.is-visible {
			visibility: visible;
			opacity: 1;
			transition: visibility 0s linear 0s, opacity $animation-timing;
		}

		&:hover {
			background-color: var(--studio-white);
			color: var(--studio-black);
			border-color: rgba(0, 0, 0, 0.1);
		}
	}

	&:not(.is-mobile) {
		.back-to-top-button__help-center {
			padding: 6px 0;
		}
	}
}