[e.cash] Stealth theme tweaks

Summary:
A few theme adjustments to the stealth theme I thought look more stealthy.
Also making it the default theme so can focus more on it without having to switch

Test Plan: preview the site and check it looks good

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14256