wp-includes/css/classic-themes.css

/**
 * These rules are needed for backwards compatibility.
 * They should match the button element rules in the base theme.json file.
 */
.wp-block-button__link {
	color: #ffffff;
	background-color: #32373c;
	border-radius: 9999px; /* 100% causes an oval, but any explicit but really high value retains the pill shape. */

	/* This needs a low specificity so it won't override the rules from the button element if defined in theme.json. */
	box-shadow: none;
	text-decoration: none;

	/* The extra 2px are added to size solids the same as the outline versions.*/
	padding: calc(0.667em + 2px) calc(1.333em + 2px);

	font-size: 1.125em;
}

.wp-block-file__button {
	background: #32373c;
	color: #ffffff;
	text-decoration: none;
}