[Cashtab] Fix hover effect for svg buttons

Summary: Want a hover effect for these. A little inconsistent as some svgs have color determined primarily by path, others primarily by fill. But this combo works for the ones in Cashtab.

Test Plan:
npm start and review, or check screenshots here

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16922