style.css

html, body {
	padding: 0;
	margin: 0;
}
body, input {
	font: large/normal monospace;
}
body {
	height: 100vh;

	display: grid;
	grid-template-rows: 3.5rem 1fr;
}
h1 {
	font-size: 1.5rem;
	font-weight: normal;
	padding: 1rem;
	margin: 0;
}
.converter {
	display: flex;
	align-items: center;
	justify-content: center;
	flex-direction: column;
}
.converter section {
	margin-bottom:1.5rem;
	display: flex;
	flex-direction: column;
}
.converter section:first-child {
	margin-bottom: 3rem;
}
.converter label {
	margin-bottom: 0.5rem;
}
.converter label::after {
	content: ": ";
}
.converter input {
	background: transparent;

	border-width: 0 0 0.15rem 0;
	border-style: solid;
	border-color: rgb(0, 0, 0, 0.2);
}

@media (min-width: 32rem) {
	.converter section {
		flex-direction: row;
	}
	.converter label {
		width: 7rem;
		margin-bottom: 0;

		border-width: 0 0 0.15rem 0;
		border-style: solid;
		border-color: rgb(0, 0, 0, 0.2);
	}
	.converter input {
		width: 26ch;
	}
}