Against my tenet to hide all the modern default themes, I was planning to add a custom theme that I generated in the Fabric UI Theme Generator. Initially I was able to apply the new custom theme and it showed up, but when I removed it to rename it and reapply it disappeared and only showed default theme from Classic look. When I tried to restore the default themes they also no longer show up.
I had opened a ticket with Microsoft O365 Support and originally they were not able to help me, but now I've received word that there is a service interruption going on regarding seeing modern themes. I was able to access them by going to the Style Library on any given site as a work around (at least as of now)