Use ${resURL} for ${attrs.css}.

core/src/main/resources/lib/layout/layout.jelly

@@ -94,7 +94,7 @@ ${h.initPageVariables(context)}
     <link rel="stylesheet" href="${resURL}/css/style.css" type="text/css" />
     <link rel="stylesheet" href="${resURL}/css/color.css" type="text/css" />
     <j:if test="${attrs.css!=null}">
-      <link rel="stylesheet" href="${rootURL}${attrs.css}" type="text/css" />
+      <link rel="stylesheet" href="${resURL}${attrs.css}" type="text/css" />
     </j:if>
     <link rel="shortcut icon" href="${resURL}/favicon.ico" type="image/vnd.microsoft.icon" />