[Cashtab] Migrate some css to styled components

Summary:
Cashtab still relies on some hard coded css to effectively integrate some components from antd. Best case scenario, Cashtab would have all of its own custom components. However, there is value in using antd with some customizations for rapid prototyping.

The purpose of this diff is to commence migrating as much css as possible away from pure css and into styled components. The end goal is to allow different themes enabled by user settings or tokens, e.g. dark mode.

This diff removes dead css and commented out css. It also migrates the .App css to the CustomApp styled component. Future diffs will take on one part of the app at a time.

Test Plan: npm test to confirm unit tests still pass. npm start and check the main app screens for any css errors.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9285