[Cashtab] Replace hardcoded colors with references to theme.js

Summary: Replace all hardcoded colors with references to theme.js. Related to task T2137.

Test Plan:
cd web/cashtab && npm start
use 'color' as a search term in ide
ensure each component that used hardcoded colors has been updated to use references to theme.js
ensure that colors are displaying properly within these components.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10825