Task/gh 191 support light navbar for dark logos remove api solution

[wesleyboar]

Overview

Update #192 to not need an update to Portal nor Docs.

Changes

• New: Document .env file via a .env.sample.
• New: Support native* built-time variables a.k.a. environment variables—env().
• New: Use built-time variables to apply different theme.
• New: Delete API.
• New: Drop THEME from secrets.py.
• New: Read THEME from .env.
• New: Update s-header.css to use environment variables—env().
• New: Improve site.header.css management of global variables a.k.a. custom properties—var().
• New: Restore logic extracted from importHTML.js.
• New: Delete site.css.js, setting t-{{THEME}} in template, and the urls.py API entry.

* Though the specification to assign environmental variables is far from complete, the implementation to use environmental variables (i.e. using env(…) and a --* var name prefix just like for var(…)) has already been supported in all modern browsers. We already have installed a plugin postcss-preset-env that includes the polyfill to assign environmental variables (it is postcss-env-function).