example.css

:root {
	--body-background: #fff;
	--base-font-color: #000;
	--active-color: blue;
	--outline-color: blue;
	--dialog-height: 200px;
	--dialog-shift-top: calc(50% - (var(--dialog-height) / 2));
	--dialog-z-index: 1;
	--char-multiplication-sign: '\00D7';
	--bg-overlay-color: rgba(17, 13, 13, 0.5);
}

/* html {
	box-sizing: border-box;
}

*,
*::before,
*::after {
	box-sizing: inherit;
}

body {
	background: var(--body-background);
} */

dialog {
	display: block;
	padding: 0;
	top: var(--dialog-shift-top);
	visibility: hidden;
	opacity: 0;
	transition: opacity 0.5s;
}

dialog[open] {
	visibility: visible;
	opacity: 1;
}

dialog>div {
	background: inherit;
	position: relative;
	padding: 1rem;
	min-height: var(--dialog-height);
	z-index: calc(var(--dialog-z-index) + 1);
}

dialog>button {
	position: fixed;
	text-decoration: none;
	top: 0;
	left: 0;
	height: 100%;
	width: 100%;
	border: none;
	cursor: pointer;
	padding: 1rem;
	background: var(--bg-overlay-color);
	z-index: var(--dialog-z-index);
	font-size: 0;
	color: transparent;
}

dialog>button:focus::before {
	outline: none;
	box-shadow: 0 0 0.2em 0.3em var(--outline-color);
}

dialog>button::before {
	font-family: Verdana, Geneva, sans-serif;
	content: var(--char-multiplication-sign);
	font-weight: bold;
	background: var(--body-background);
	border: 2px solid var(--bg-overlay-color);
	position: absolute;
	top: 1.5rem;
	right: 1.5rem;
	display: block;
	color: var(--base-font-color);
	font-size: 1.2rem;
	width: 2rem;
	height: 2rem;
	border-radius: 50%;
	line-height: 1.6rem;
	text-align: center;
	transition: border-color 0.5s, color 0.5s, box-shadow 0.1s;
}

dialog>button:focus::before,
dialog>button:hover::before {
	border-color: var(--body-background);
	color: var(--active-color);
}