Hi Mark,
Right now, you can’t change the colour scheme in code, but I’ll add it to our list.
Technically, you could update the color scheme dropdown menu by editing the app’s yaml file, but the color schemes won’t work with the original Material Design’s theme.css. You’d have to also copy and paste Material Design 3’s theme.css and standard-page.html files. You can clone a new Material Design 3 app locally to copy the yaml file.