src/vs/workbench/services/themes/common/colorThemeData.ts/toCSSSelector
function toCSSSelector(extensionId: string, path: string) {
	if (path.startsWith('./')) {
		path = path.substr(2);
	}
	let str = `${extensionId}-${path}`;

	//remove all characters that are not allowed in css
	str = str.replace(/[^_a-zA-Z0-9-]/g, '-');
	if (str.charAt(0).match(/[0-9-]/)) {
		str = '_' + str;
	}
	return str;
}