Wikipedia:Book sources/styles.css

/* {{pp-protected|small=yes}} */
.wp-book-sources-collapse .mw-collapsible-toggle {
	background-color: var(--background-color-neutral-subtle, #efefef);
	border: 2px solid #fff;
	border-bottom: 1px solid #36b;
	padding: 5px;
	float: none;
}

.wp-book-sources-collapse .mw-collapsible-content {
	border: 4px solid #fff;
	background-color: var(--background-color-neutral-subtle, #f7f7f7);
	padding: 10px;
}

@media (min-width: 720px) {
	.wp-book-sources-other-langs {
		min-width: 35em;
	}
}