[Cashtab] Form elements to themed styled components

Summary: Add custom form elements to new styled component theme to support easier appearance customization in future.

Test Plan: npm start; compare Send and SendToken pages with those at https://cashtabapp.com/; forms should look identical

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9312