Only the download icon is higlighted in hover state

[eloquence]

(Originally captured by @creviera in #795):

Here is what you see when you mouse over a "Download" icon or label:

This is a bug; both the icon and the label should be highlighted.

And, if the file is missing and needs to be redownloaded, the icon remains in a highlighted state even without mousing over it:

[ntoll]

Hmmm... I'm sure this was fixed in an earlier PR. I guess it's a regression. Will take a look.

[ntoll]

OK... the CSS isn't getting picked up. Not sure why, but will figure something out.

[ntoll]

Here's the problem line: https://github.com/freedomofpress/securedrop-client/blob/master/securedrop_client/gui/widgets.py#L2015 PR coming in a moment.