Bugfix

Found the bug. 
fs.createReadStream(themePath).pipe(fs.createWriteStream(options.themeImport));

pipe is async, so you need to wait the end of the first read/write if you want if you want to redo a read/write to the same files.

Just wrapping everything on a <readstream>.on('end'. function(){}) seems to solve the problem. Now it should be ok!