[Cashtab] Move all colors to theme

Summary:
Move css styles to theme for easier control of app styles.

This is a prereq for features like dark mode. It will also make a rebrand design much more straightforward.

It looks like a bunch of changes, but it's only replacing css colors in styled components with variables. The key file to review is themes.js. Snapshots and unit tests had to be updated to support the new ThemeProvider component.

Test Plan: npm start and compare to cashtabapp.com; styling should be identical except some dark gray text becomes black

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9322