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
<xen:include template="xendisConnect.css" />
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.