add genericon to sidebar header for custom widget

View my site (and the problem) here.

In the sidebar there are widgets: the Ephemera widget from Twenty Fourteen, mildly adapted, followed by my custom Facebook likebox widget below, then another Ephemera widget with different content.

As you can see, the Ephemera widget creates a gret box to the left of the header and places a Genericon in it, one for each type of Ephemera display.

The Genericon set contains icons for Facebook, and I’d like to put a similar grey box next to the Facebook widget title and display the Facebook genericon within.

The problem is it’s the sidebar, not the Facebook widget, that generates that header and I don’t have control of it. So… how would I go about telling to to make a grey box for all sidebar widget headers, and to put genericons in the ones it recognises?

Twenty Fourteen gurus? Li’l help?