Module:collapsible category tree/style.css

/* Styles applying equally to collapsible and non-collapsible instances */
.columns-bg.term-list.CategoryTreeTag {
	margin-top: 0;
	margin-bottom: 0;
}

/* suppress default appearance of triangular toggle button */
.columns-bg.term-list.CategoryTreeTag .CategoryTreeToggle {
	display: none;
}
.columns-bg.term-list.CategoryTreeTag .CategoryTreeBullet::after {
	content: "▼";
}

/* Make the list switcher one line taller than default */
.list-switcher-category-tree {
	bottom: calc(4.2 * 1.6em); /* can't use "lh" units in TemplateStyles */
}
.list-switcher-category-tree.list-switcher-collapsed {
	max-height: calc(4.2 * 1.6em); /* can't use "lh" units in TemplateStyles */
}

/* The objective of the next few rules is to make CategoryTree's <div> output
   look indistinguishable from a default MediaWiki bulleted list. */
.list-switcher-category-tree .CategoryTreeChildren {
	column-count: 3;
	/* match default MediaWiki styling of <ul> */
	margin-left: 1.6em;
	margin-right: 0;
	padding: 0;
}

.list-switcher-category-tree .CategoryTreeChildren .CategoryTreeSection {
	/* match default MediaWiki styling of <li> */
	display: list-item;
	margin-bottom: 0.1em;
}

.list-switcher-category-tree .CategoryTreeChildren .CategoryTreeItem {
	display: inline;
}

.list-switcher-category-tree .CategoryTreePageBullet {
	display: none;
}

/* hack - proof of concept */
.list-switcher-category-tree > .CategoryTreeTag:not([data-pages-left-over="0"]) .CategoryTreeChildren::after {
	content: "...and " counter(pagesleftover) " more";
}