define custom properties into :root

lib/single-file/single-file-core.js

@@ -903,7 +903,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 				}
 			}));
 			if (indexVariable) {
-				stylesheetContent = "html{" + stylesheetContent;
+				stylesheetContent = ":root{" + stylesheetContent;
 			}
 			return stylesheetContent;
 		}