Round corners of images in portfolio - floating

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How can I round the corners too 20px on the images in my portfolio. 

They appear in the background when hover over the project name - see screen-recording attached. 

Thanks! 

Please see the following.

Let us know how it goes.

I can't seem to make it work - I have already used a few of these for my product pages, however this is a portfolio page for selected work? Does that help? 

Here is what I see when I install my code locally, set the settings, and then hover.

The settings I used.

  @border-color : unset;
  @border-radius : 20px;
  @border-style : solid;
  @border-width : 0px;