src/vs/base/browser/ui/tree/abstractTree.ts/AbstractTree/style
class AbstractTree:	style(styles: IListStyles): void {
		const suffix = `.${this.view.domId}`;
		const content: string[] = [];

		if (styles.treeIndentGuidesStroke) {
			content.push(`.monaco-list${suffix}:hover .monaco-tl-indent > .indent-guide, .monaco-list${suffix}.always .monaco-tl-indent > .indent-guide  { border-color: ${styles.treeIndentGuidesStroke.transparent(0.4)}; }`);
			content.push(`.monaco-list${suffix} .monaco-tl-indent > .indent-guide.active { border-color: ${styles.treeIndentGuidesStroke}; }`);
		}

		this.styleElement.textContent = content.join('\n');
		this.view.style(styles);
	}