1) There's no mention of where to set the theme name. An example has THEME_NAME, but that is an obsolete setting. The correct setting is DEFAULT_SITE_THEME.
2) The instructions mention editing lms.env.json (et al.), but that file is only read if you use --settings=aws, and if you do that on a devstack, then the theme doesn't work. This gets into the larger issue of the complexity of where to change settings. EXTRA CREDIT: see where we try to explain this, and let's make it better.