neat.css

:root {
	color-scheme: dark light;
	--light: #fff;
	--dark: #404040;
}

* {
	box-sizing: border-box;
	color: var(--dark);
}

html {
	border-style: solid;
	border-width: 5px 0 0 0;
	border-color: var(--dark);
}

html, button, input {
	font-family: sans-serif;
}

input {
	border: 1px solid silver;
	border-radius: 5px;
	padding: 5px; 
}

body {
	background-color: var(--light);
	margin: 0;
	max-width: 800px;
	padding: 0 20px 20px 20px;
	margin-left: auto;
	margin-right: auto;
}

p {
	margin-bottom: 10px;
	line-height: 1.4em;
}

img {
	width: 100%;
}

pre {
	overflow: auto;
}

button, .button, input[type=submit] {
	display: inline-block;
	background-color: var(--dark);
	color: var(--light);
	font-size: 1em;
	text-align: center;
	padding: 8px;
	margin-right: 5px;
	border-radius: 5px;
	text-decoration: none;
	border: none;
	line-height: normal;
	cursor: pointer;
}


button:last-child, .button:last-child {
	margin-right: 0;
}

.center {
	display: block;
	margin-left: auto;
	margin-right: auto;
	text-align: center;
}

.bordered {
	border: 3px solid;
}

.home {
	display: inline-block;
	background-color: var(--dark);
	color: var(--light);
	margin-top: 20px;
	padding: 5px 10px 5px 10px;
	text-decoration: none;
	font-weight: bold;
}


/* Desktop sizes */
@media only screen and (min-width: 600px) {
	ol {
		column-count: 2;
	}
	
	.row {
		display: flex;
		flex-direction: row;
		padding: 0;
		width: 100%;
	}
	
	.row .column {
		display: block;
		flex: 1 1 auto;
		max-width: 100%;
		width: 100%;
	}
	
	.row .column:not(:last-child) {
		margin-right: 10px;
	}
}

/* Dark mode overrides */
@media (prefers-color-scheme: dark) {
	* {
		color: var(--light);
	}
	
	html {
		border-color: var(--light);
	}
	
	body {
		background-color: var(--dark);
	}
	
	button, .button, .home, input {
		background-color: var(--light);
		color: var(--dark);
	}
}