I have created a new style by copying an existing style in the style list in client\stemapp\themesFoldableTheme\manifest.json and changed the name, description and styleColor properties. Then I copy and paste one of the folders in \client\stemapp\themes\FoldableTheme\styles naming it with the same name I added to the styles list. Then I make whatever color changes in the style.css in my new style folder.
Jeff Ward
Summit County, Utah