In addition to eneubauer’s comment:
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It could be a permission problem as well, make sure your theme directory is readable by your www-user.
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Be very conservative with your theme ID (=directory name). I’d recommend to only use the syntax
theme-simplename
Use lowercase characters only and do not use any special characters.
I also ran into issues when the name was too long, so keep it short
I didn’t dig into the code, but from the past I know that e.g. underscore characters in the theme ID will prevent it from being enabled. Just use theme-simpleletters