So, those theme_url + '/assets/css/style-wide.css' lines in your init.js are referencing a theme_url variable that is defined on line 36 of alpha/templates/default.html.twig
That value is set to your_site_folder/user/themes/mytheme, which means all those CSS files (your_site_folder/user/themes/mytheme/assets/css/style-wide.css etc), can't be loaded because we can't handle that part with Grav's streams.
You have two options here:
- copy all those css files into your inherited theme (so
your_site_folder/user/themes/mytheme/assets/css/style-wide.css etc do exist)
- copy the
alpha/templates/default.html.twig to your theme and manipulate the theme_url variable directly.
Example code for option two would be (l24-27):
<script>
var base_url = '{{ base_url }}',
theme_url = '{{ theme_url }}'.replace('/mytheme', '/alpha');
</script>