chore: retire env()-based themes (Core-CMS v3.9)

[wesleyboar]

Overview

Replace env()-based themes with simpler, immediate solutions.

Related

• required by https://github.com/TACC/Core-CMS/pull/632

Changes

1. Migrate ECEP theme env()s to ECEP site.css as var()s; follow-up:
   • none, because ECEP site.css already loads
2. Delete ProTX theme; follow-up:
   • [x] add snippet to pprd.protx that mimics theme.has-dark-logo.css[^1]
   • [x] add snippet to prod.protx that mimics theme.has-dark-logo.css[^1]
3. Delete TUP-CMS theme; follow-up:
   • none, because this project folder is unused

[^1]: This is bad, because snippets load slower and can be forgotten. This seems a reliable solution short-term, because ProTX does not have a site.css; and adding one means adding a new template, then—after deploy—changing the template every page loads (because pre–TACC/tup-ui per-project site.css solution was poorly engineered by @wesleyboar).

Testing & UI

See https://github.com/TACC/Core-CMS/pull/632.