[e.cash] Add styled components

Summary:
Adding the styled components to the site to help with theme based styling
Created two theme objects for ecash and stealth colors
Added a theme toggle for use in the development process
Updated the current components to styled components styling using the new themes

Test Plan:

• Either run the app locally with npm run dev or generate a preview build with @bot preview-ecash
• Inspect the page to ensure everything functions and looks as before
• click the theme switcher box in the bottom right to toggle between themes

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: Fabien, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D13770