I took the initial steps to create a custom theme for a D8 site (basically, copying the stark theme and changing the folder name and the YAML files)
When I went to the /admin/appearance page, I initially had an error due to a syntax issue in the info.yml file (an accidental extra quote mark in the description). I fixed that and the theme showed up on the Appearance page. However, when I tried to enable the theme, I received a message that the theme {themename} could not be found.
Once I manually cleared all the caches, I could enable the theme as normal.
(As an additional note, my theme name initially had a hyphen, but I couldnt see it in the /admin/appearance page until I removed the hyphen)