Moving bcha page CSS to main.css

Summary:
The override.css file was not the ideal place to put the CSS for the new bcha page. Was not rendering live.
Moved the new rules to main.css instead

Test Plan: Make sure page is rendering properly live

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8484