As I understood it, this would amend extra.css and not replace the contents of what may already be there in a custom style. I learned recently that this does not always work (link). In this example, the following code was already in the style's extra.css PHP: <xen:include template="xendisConnect.css" /> But what ended up happening was TMS was not amending the template it was replacing it. And directly editing the template outside of TMS also did not change anything. I could put any value or code I wanted directly into extra.css, but it was useless. TMS was completely replacing the full context of the template, when it should only have amended it.