Custom theme exposes unformatted Opigno menu

jmev
I created a subtheme and upon activating it, my main page instantly exposes the Opigno menu with no formatting. I'm not sure what I'm supposed to do. Here's what the Opigno pages look like now: https://postimg.cc/pmV0P7MB What do I need to do to properly format the menu?
jmev

The sidebar-first is not hidden in custom subtheme

I noticed that what is happening is that the 'sidebar-first' region is visible in the custom theme, but in the default theme, Aristotle, it is missing from the markup, leading me to ask if this could be intentional and managed via code. I'll see if I can find code that would intentionally hide this region. However, I would expect the subtheme to inherit all functionality and styling from, and work exactly like the parent theme until it is overridden, so this seems to be a bug. I've also opened an issue on the project page, located here: https://www.drupal.org/project/opigno_lms/issues/3247297