I suspect that any searchable term can be used in the “Tags” header in the File Header for a theme.
The codex discusses the File Header Requirements here https://codex.wordpress.org/File_Header but does not explain how to choose which values to assign to “Tags”.
You can add any value you want to the “Tags” header. You are limited only if you want your theme be listed in some web. In that case, you must follow the restrictions that the owner/community of that web has set.
For example, if you want your theme be listed in WordPress.org themes directory, you must follow the requirements published by the Theme Review Team. You can find the guidelines for theme tags here.