darkfish raises with --template-stylesheets option

[scivola]

reproduction code

rdoc --template-stylesheets hoge.css --debug

• ruby 3.0.2
• rdoc 6.3.2
• fileutils 1.5.0

result

wrong number of arguments (given 3, expected 2)

cause

https://github.com/ruby/rdoc/blob/master/lib/rdoc/generator/darkfish.rb#L224

FileUtils.cp has two positional arguments and three keyword arguments at version 1.5.0 :
https://github.com/ruby/fileutils/blob/master/lib/fileutils.rb#L429

[nobu]

No tests for --template-stylesheets option since #205.