[Cashtab] migrating more form css to styled components

Summary: Migrating more color-dependent css out of App.css and into styled components

Test Plan: npm start; compare forms on Send page with those on Send page of cashtabapp.com; compare "Import Mnemonic" form and wallet rename/delete forms from Settings page with those on cashtabapp.com; open in incognito window and compare that import mnemonic form to those at cashtabapp.com; should all be identical

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9315