I've made a fresh install of SS 3.1.2 which ran without any problems until I came to edit a theme.
As in previous versions I took a copy of Simple theme and gave the folder a different name and uploaded to the server having changed the theme name in the composer.json file ...
The new theme appears in the "Settings" tab in the back end, BUT, if I change the theme to the copy of Simple the site loses all of the Black & White colouring of the Simple theme and shows a blue bar at the top with the page name and the site "content" underneath.
Clearly it's not picking up the newly selected theme but rather falling back to a much simpler theme.
I've tried all sorts trying to make this work - wiped the themes directory clean and re-uploaded, recopied the simple theme to another folder (With the same result), changed the theme name in _config.php and _config.yml.
Setting the site into "dev" mode doesn't show any errors.
I'm at a loss - can anyone help or is this a serious bug in the new release?
Many thanks - Jeremy