the using system and attribute don't appear in their right color instead they appear in light blue. what does that mean? : [class Book[\]\[1\]][1]

[class program][2]


  [1]: https://i.stack.imgur.com/WeOOb.png
  [2]: https://i.stack.imgur.com/nfGv8.png
this is the code, I've tried rewriting it in a new program but its the same problem as soon as I create a class the "using system" and the others appear in light blue as well as attributes 