[e.cash] Remove theme switch

Summary:
Will be no need for the theme switch in prod, and need to make room for language switcher
Removing it here and setting theme to stealth

Test Plan: preview the site and see if all looks right

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D14280