<p>Ok, following is the result with your <code>1px border</code> suggestion.<br>
<a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/617017/60686960-2a66a400-9e9b-11e9-94a2-37f3105971ec.png"><img src="https://user-images.githubusercontent.com/617017/60686960-2a66a400-9e9b-11e9-94a2-37f3105971ec.png" alt="website_themes_rounded_corners_border" style="max-width:100%;"></a></p>
<p>I think this is a bit broad border. I tried using a <code>box-shadow</code> instead:<br>
<a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/617017/60686986-49fdcc80-9e9b-11e9-86ee-432b17550efd.png"><img src="https://user-images.githubusercontent.com/617017/60686986-49fdcc80-9e9b-11e9-86ee-432b17550efd.png" alt="website_themes_rounded_corners_shadow" style="max-width:100%;"></a></p>
<p>Code used: <code>box-shadow: 0px 0px 3px 1px rgba(0, 0, 0, 0.5);</code></p>

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