[Cashtab] Remove all colors from App.css

Summary: Remove last of color refs from App.css. Remove styled component import mod styled-components/macro after testing revealed it was not necessary.

Test Plan: npm start; compare to cashtabapp.com, confirm forms on Send page are identical including orange highlighting on currency dropdown; try to delete a wallet, confirm modal buttons in confirm delete pop-up are styled identically

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9321