nicecolor.js/demo/assets/css/style.css

/* Global */

:root {
	--theme-color: #212121;
	--theme-padding: 8vh;
	--transition-speed: 300ms;
}

* {
	font-family: "Roboto Mono","sans-serif";
	margin: 0;
}

*::selection {
	background: var(--theme-color);
	color: white;
}

.flexCenter {
	display: flex;
	justify-content: center;
	align-items: center;
}

/* main */

main {
	width: 100vw;
	height: 100vh;
	display: flex;
	overflow: hidden;
}

main .block {
	width: 50%;
	height: 100%;
}

main .block:first-child {
	transform: translate(-50vw);
}

main .block:last-child {
	transform: translate(50vw);
}

main.active .block {
	transform: translate(0);
}

main .block > div {
	width: calc(100% - var(--theme-padding) * 2);
	height: calc(100% - var(--theme-padding) * 2);
}

/* main > #pattern */

main #pattern {
	background: white;
}

main #pattern .birdplane {
	--size: 10vw;
	--rotation: -45deg;
	position: absolute;
	top: 25vh;
	left: 0%;
	width: 0;
	height: 0;
	border-left: calc(var(--size) * 2) solid transparent;
	border-right: calc(var(--size)) solid transparent;
	border-top: calc(var(--size) / 2) solid var(--theme-color);
	transform-origin: 100% 0%;
	transform: rotate(var(--rotation));
	opacity: .13;
	animation: wingLeft 3s infinite alternate ease;
}

main #pattern .birdplane:last-of-type {
	transform: rotate(calc(var(--rotation) * 1.9)) scaleY(-1);
	animation: wingRight 3s infinite alternate ease;
}

@keyframes wingLeft {
	to {
		transform: rotate(calc(var(--rotation) + 2deg));
	}
}

@keyframes wingRight {
	to {
		transform: rotate(calc(var(--rotation) * 1.95)) scaleY(-1);
	}
}

/* main > #controls */

main #controls {
	background: var(--theme-color);
}

main #controls #text {
	color: white;
	text-align: center;
}

main #controls #text *::selection {
	background: white;
	color: var(--theme-color);
}

main #controls #text h1 {
	font-size: 9vh;
	text-transform: uppercase;
}

main #controls #text h2 {
	font-size: 5vh;
	font-weight: normal;
}

main #controls #text h2:last-of-type {
	width: 70%;
	font-size: 3vh;
	margin-left: auto;
    margin-right: auto;
}

main #controls #pick {
	margin-top: auto;
}

@media screen and (max-width: 1000px) {
	main {
		flex-wrap: wrap;
	}

	main .block {
		width: 100%;
		height: 50%;
	}

	main #pattern .birdplane {
		--size: 22vw;
		top: 20vw;
	}
}