[cashtab] Cleanup theme file

Summary:
The theme got a little wild to maintain or easily swap values to create other theme options
With the redesign the color pallete was simplified, so cleaning up the theme here to remove unused properties

Test Plan: npm start and check nothing off color wise

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17296