[Cashtab] Introduce CSS variables to make styling easier

Summary:
In the original font-size hardcoded styles, there were 11 different font sizes. CSS variables were introduced to standardize writing in a more
readable way, which will have long-term benefits for typography. Line height was also added for <p> tags.

Test Plan: npm test

Reviewers: emack, bytesofman, O1 Bitcoin ABC, #bitcoin_abc, johnkuney

Reviewed By: bytesofman, O1 Bitcoin ABC, #bitcoin_abc, johnkuney

Subscribers: johnkuney

Differential Revision: https://reviews.bitcoinabc.org/D17815