[e.cash] Move some pages styles to a common place

Summary:
In preparation for making a new page and re-using some of these styles.
This is only moving things, and should not change anything on the website.

Test Plan: Run it locally, looks at core-tech page to see that it lokos the same

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16172