src/vs/workbench/services/themes/browser/workbenchThemeService.ts/_applyRules
function _applyRules(styleSheetContent: string, rulesClassName: string) {
	const themeStyles = document.head.getElementsByClassName(rulesClassName);
	if (themeStyles.length === 0) {
		const elStyle = document.createElement('style');
		elStyle.type = 'text/css';
		elStyle.className = rulesClassName;
		elStyle.textContent = styleSheetContent;
		document.head.appendChild(elStyle);
	} else {
		(<HTMLStyleElement>themeStyles[0]).textContent = styleSheetContent;
	}
}