demo.css

body {
	--bg-1: hsl(0, 0%, 100%);
	--bg-2: hsl(206, 20%, 90%);
	--bg-3: hsl(206, 20%, 80%);
	--fg-1: hsl(0, 0%, 13%);
	--fg-2: hsl(0, 0%, 20%);
	--fg-2: hsl(0, 0%, 30%);
	--link: hsl(208, 77%, 47%);
	--link-hover: hsl(208, 77%, 55%);
	--link-active: hsl(208, 77%, 40%);
	--border-radius: 4px;
	--font: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell,
		'Open Sans', 'Helvetica Neue', sans-serif;
	--font-mono: ui-monospace, 'Cascadia Code', 'Source Code Pro', Menlo, Consolas, 'DejaVu Sans Mono',
		monospace;
	background: var(--bg-1);
	color: var(--fg-1);
	font-family: var(--font);
	line-height: 1.5;
	margin: 1rem;
	height: calc(100vh - 2rem);
}

h1,
h2,
h3,
h4,
h5,
h6 {
	font-weight: normal;
	font-variant-numeric: tabular-nums;
	line-height: 1.1;
}

:is(h1, h2, h3, h4, h5, h6, p) {
	margin: 1rem 0.1rem;
}

label {
	margin: 0.5rem 0.1rem;
}

:is(h1, h2, h3, h4, h5, h6, p, label):first-child {
	margin-top: 0;
}

:is(h1, h2, h3, h4, h5, h6, p, label):last-child {
	margin-bottom: 0;
}

a {
	color: var(--link);
}

a:hover {
	color: var(--link-hover);
}

a:active {
	color: var(--link-active);
}

label {
	display: flex;
	gap: 0.5rem;
	align-items: center;
}

label input {
	margin: 0;
}

button,
input,
select {
	font-family: inherit;
	font-size: inherit;
}

button {
	background: var(--link);
	color: var(--bg-1);
	padding: 0.5rem 1rem;
	border: none;
	border-radius: var(--border-radius);
}

button:hover {
	background: var(--link-hover);
}

button:active {
	background: var(--link-active);
}

:is(button, button:hover, button:active):disabled {
	background: var(--link);
	filter: grayscale(1);
	opacity: 0.4;
}

input,
textarea,
select {
	padding: 0.5rem;
	border: 1px solid var(--bg-2);
	border-radius: var(--border-radius);
	box-sizing: border-box;
}

input,
textarea {
	background: var(--bg-1);
}

select:not([multiple]) {
	background: var(--bg-2);
}

textarea {
	font-family: var(--font-mono);
	font-size: 0.9rem;
}

form {
	display: flex;
	flex-direction: column;
	gap: 1rem;
	align-items: baseline;
}

ul:has(li):has(form) {
	list-style: none;
	padding: 0;
}

li form {
	flex-direction: row;
	gap: 0.5rem;
	margin: 0.5rem 0;
}

nav {
	position: relative;
	display: flex;
	gap: 1em;
	padding: 1em;
	background: var(--bg-2);
	z-index: 2;
	margin: 0 0 1em 0;
	border-radius: var(--border-radius);
}

nav a {
	text-decoration: none;
}

nav a[aria-current='true'] {
	border-bottom: 2px solid;
}

ul:has(form) {
	list-style: none;
	padding: 0;
}

progress {
	margin: 0.5rem 0;
}

progress:first-child {
	margin-top: 0;
}

progress:lsat-child {
	margin-bottom: 0;
}

.error {
	color: red;
}

code {
	background: var(--bg-2);
	font-family: var(--font-mono);
	font-size: 0.9em;
	padding: 0.15rem 0.3rem;
	border-radius: var(--border-radius);
}

ul.todos {
	padding: 0;
}

ul.todos li:not(:has(> form)),
ul.todos li form {
	position: relative;
	display: flex;
	align-items: center;
	padding: 0.5em 0.5em 0.5em 1em;
	margin: 0 0 0.5em 0;
	gap: 0.5em;
	border-radius: 5px;
	user-select: none;
	background: var(--bg-1);
	filter: drop-shadow(2px 3px 6px rgba(0, 0, 0, 0.1));
	transition:
		filter 0.2s,
		opacity 0.2s;
}

ul.todos .done {
	filter: none;
	opacity: 0.4;
}

ul.todos button {
	border: none;
	background-color: transparent;
	background-repeat: no-repeat;
	background-position: 50% 50%;
	background-size: 1rem 1rem;
	cursor: pointer;
	width: 3em;
	height: 3em;
	margin: -0.5em -0.5em -0.5em 0;
	aspect-ratio: 1;
	opacity: 0.5;
	transition: opacity 0.2s;
}

ul.todos button:hover {
	opacity: 1;
}

body.dark {
	--bg-1: hsl(0, 0%, 18%);
	--bg-2: hsl(0, 0%, 30%);
	--bg-3: hsl(0, 0%, 40%);
	--fg-1: hsl(0, 0%, 90%);
	--fg-2: hsl(0, 0%, 70%);
	--fg-3: hsl(0, 0%, 60%);
	--link: hsl(206, 96%, 72%);
	--link-hover: hsl(206, 96%, 78%);
	--link-active: hsl(206, 96%, 64%);
}