Is it possible to take a screenshot of a dashboard in OS X?
I'm assuming you set up  Cmd ⌘   Shift ⇧   5  yourself, as it's not a System default.

If you use  Cmd ⌘   Shift ⇧   4  then you can drag a marquee round the item you want, or even use  Space  afterwards to grab the entire floating widget, examples below…


Using  Cmd ⌘   Shift ⇧   4 
Note the hard edges &amp; visible background.

Using  Cmd ⌘   Shift ⇧   4   followed by  Space  then clicking the desired 'window'/widget.
Note the soft edges with shadows