Remove the plugin_theme metadata field. Not useful.

lib/theme_template/theme.gemspec.erb

@@ -10,8 +10,6 @@ Gem::Specification.new do |spec|
   spec.homepage      = "TODO: Put your gem's website or public repo URL here."
   spec.license       = "MIT"
 
-  spec.metadata["plugin_type"] = "theme"
-
   spec.files         = `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(exe|<%= theme_directories.join("|") %>)/}) }
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }