A theme change compiled Bootswatch with libsass (34 MB, upstream
deprecated) into two tracked files, which updates, rollbacks and new
Docker containers then reset. The 27 themes now ship as
static/css/themes/<theme>.min.css, built by "manage.py build_themes";
the Settings page only saves the theme name and every page links that
file (zephyr when no settings are in context, as on the 500 page).
libsass moves to dev/requirements.txt; a test checks that each shipped
file is its theme compiled from dev/scss. The "Theme SASS Path" setting
is removed, and the theme default "flaty", a typo that never applied,
becomes zephyr, the theme those installs showed. The update and
rollback steps restore an older version's rewritten theme files only
where they are modified.
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