This project has all the css code in one giant file and there are 
  pros and cons of doing this.

  pros: 
    slightly faster webpage load times and less network traffic.
  cons: 
    you have one big giant css file which can be hard to maintain if the
    project is just a little bit larger than this.
  
  In the industry, we separate the css codes into multiple files. 
  The css code can easily be thousands of lines in a small real-world projects
  and we have no choice but to separate the codes. 
  By doing this we get the benefit of maintainability.

  When the code is actually sent to the clients/users there are tools
  to combine the css(and JavaScript) code into less css file. 
  By doing this we get the benefit of less network traffic and slightly 
  faster load times.

  Do not worry about it for now if this does not make sense.
  Try different approaches and see how you like it.
  You will learn/experience more about this throughout the journey of IWD.