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CSS

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HTML 

              
                mixin split(string)
	- var array = string.split('')
	- for (var i = 0; i < array.length; i += 1) 
		span #{array[i]}

div#mode-circle
#blob.blob
.title
	h1
		span
			+split('Dolore')
		span
			+split('Magna')
footer
	button#toggle-mode
		svg(xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none")
			path(id="icon" stroke-linejoin="round" stroke-width="1.5" stroke="currentColor" d="M2.03009 12.42C2.39009 17.57 6.76009 21.76 11.9901 21.99C15.6801 22.15 18.9801 20.43 20.9601 17.72C21.7801 16.61 21.3401 15.87 19.9701 16.12C19.3001 16.24 18.6101 16.29 17.8901 16.26C13.0001 16.06 9.00009 11.97 8.98009 7.13996C8.97009 5.83996 9.24009 4.60996 9.73009 3.48996C10.2701 2.24996 9.62009 1.65996 8.37009 2.18996C4.41009 3.85996 1.70009 7.84996 2.03009 12.42Z")
.noise
!

CSS 

              
                @import url("https://fonts.googleapis.com/css2?family=Montserrat&display=swap");
@import url("https://fonts.googleapis.com/css2?family=Fira+Sans:wght@700&display=swap");

*, *::after, *::before {
	margin: 0;
	padding: 0;
	box-sizing: border-box;
}

:root {
	--padding-container: 16;
	--max-container: 1400px;

	--font-size-title: clamp(4.05rem, 18.78vw + -1.64rem, 11.26rem);

  --bg: hsl(0, 0%, 100%);
  --text-primary: hsl(0, 0%, 10%);

	--blob-color: 33, 139, 10;
	--blob-x: 1;
	--blob-y: 1;
}

html,
body {
	height: 100%;
	font-size: 16px;
}

body {
	background-color: var(--bg);
	width: 100vw;
	height: 100vh;
	font-family: "Montserrat", sans-serif;
	color: var(--text-primary);
	overflow: hidden;
	@supports (height: 100dvh) {
		height: 100dvh;
	}
}

.blob {
	position: absolute;
	transform: translate3d(calc(var(--blob-x) * 1px), calc(var(--blob-y) * 1px), 0);
	width: 50vmin;
	height: 50vmin;
	border-radius: 50%;
	filter: blur(5vmin);
	background: radial-gradient(
		ellipse at center,
		rgba(var(--blob-color), 0.7) 0%,
		rgba(var(--blob-color), 0.65) 100%
	);
	mix-blend-mode: difference;
}
.noise {
	pointer-events: none;
	position: absolute;
	inset: 0;
	width: 100vw;
	height: 100vh;
	background: transparent url("https://assets.codepen.io/108082/Noise.png") 0 0;
	background-size: 320px 320px;
	mix-blend-mode: luminosity;
}

.title {
	display: grid;
	place-items: center;
	grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
	padding: 0 calc(var(--padding-container) * 1vmin);
	width: 100%;
	height: 100%;
	mix-blend-mode: difference;
  background: aliceblue;
	@-moz-document url-prefix() {
		mix-blend-mode: unset;
		background: transparent;
	}
	h1 {
		margin: 0 auto;
		width: min(100%, 800px);
		font: bold var(--font-size-title) "Fira Sans", sans-serif;
		text-align: center;
		letter-spacing: 1px;
		text-transform: uppercase;
		& > span {
			display: flex;
			justify-content: space-between;
		}
	}
}

#mode-circle {
	mix-blend-mode: difference;
	pointer-events: none;
	position: fixed;
	inset-block-end: 0;
	inset-inline-end: 0;
	border-radius: 50%;
	background: #fdf4f4;
	width: 20vw;
	height: 20vw;
	transform: scale(0) translate(-50%, -50%);
}

footer {
	mix-blend-mode: difference;
	position: fixed;
	inset-block-end: 32px;
	inset-inline-end: 48px;
	z-index: 2;
	button {
		background: none;
		border: none;
		min-width: 20px;
		font-size: 1.5rem;
		&:hover {
			cursor: pointer;
		}
		svg {
			color: hsl(0, 16%, 84%);
			width: 1em;
			height: 1em;
		}
		#sun, #moon {
			visibility: hidden;
		}
	}
}
!

JS 

              
                gsap.registerPlugin(MorphSVGPlugin);

const toggleModeBtn = document.querySelector('#toggle-mode');
const blob = document.querySelector("#blob");
const modeCircle = document.querySelector("#mode-circle");

const moon = "M2.03009 12.42C2.39009 17.57 6.76009 21.76 11.9901 21.99C15.6801 22.15 18.9801 20.43 20.9601 17.72C21.7801 16.61 21.3401 15.87 19.9701 16.12C19.3001 16.24 18.6101 16.29 17.8901 16.26C13.0001 16.06 9.00009 11.97 8.98009 7.13996C8.97009 5.83996 9.24009 4.60996 9.73009 3.48996C10.2701 2.24996 9.62009 1.65996 8.37009 2.18996C4.41009 3.85996 1.70009 7.84996 2.03009 12.42Z"
const sun = "M19.01 4.99L20 4M4 20L4.99 19.01M12 2.08V1M12 23V21.92M2.08 12H1M22 12H23M4.99 4.99L4 4M20 20L19.01 19.01M18.5 12C18.5 15.5899 15.5899 18.5 12 18.5C8.41015 18.5 5.5 15.5899 5.5 12C5.5 8.41015 8.41015 5.5 12 5.5C15.5899 5.5 18.5 8.41015 18.5 12Z"

const blobRect = blob.getBoundingClientRect();

const blobSizes = {
	width: blobRect.width,
	height: blobRect.height
};

const move = (event: MouseEvent) => {
	let mouseX = event.pageX - blobSizes.width / 2;
	let mouseY = event.pageY - blobSizes.height / 2;

	document.body.style.setProperty(`--blob-x`, mouseX);
	document.body.style.setProperty(`--blob-y`, mouseY);
};

const resize = () => {
	const rect = blob.getBoundingClientRect();
	blobSizes.width = rect.width;
	blobSizes.height = rect.height;
};

let isDark = false
const toggleMode = (event) => {
	event.stopPropagation()
	isDark = !isDark
	let morphSVG = isDark ? sun : moon
	gsap.to('#icon', {
		duration: 1,
		morphSVG: morphSVG,
		ease: "power1.inOut",
	});
	let animateTo = isDark ? {
		duration: 2,
		scale: 20,
		translateX: 0,
		translateY: 0,
	} : {
		duration: 1,
		scale: 0,
		translate: '-50%, -50%',
	}
	gsap.to(modeCircle, {
		...animateTo,
	})
}

window.addEventListener("mousemove", move);
window.addEventListener("resize", resize);
toggleModeBtn.addEventListener('click', toggleMode);
!
999px

Console