[RSS Feed/News] How to hide an element at certain screen widths?

[XenForo]

I have an HTML sidebar (navigation & banner ad) that I'm trying to hide at medium and smaller screen breaks.

Here's the code I'm trying to use, but it's not working:

In my PAGE_CONTAINER template I have this HTML:
Code:

<div class="narrowhide">this text should hide at medium width</div>

In my extra.less file I have this:
Code:

div.narrowhide {
  ;
}

@media (min-width: (@xf-responsiveMedium)) {
  div.narrowhide {
    display: none;
  }

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