[Cashtab] Add theming to better organize color changes

Summary: Remove color specification of .anticon class from App.css. Replace with styled-components <ThemeProvider> object and theme. For now, this is only one color affecting only the icons impacted by removing this 1 line of css. Will be expanded on so that all app styles can be centrally controlled, as with the parameters in Ticker.js.

Test Plan: npm start and confirm the icons on the Settings page and the Send page look the same as at cashtabapp.com

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9308