How to help students avoid being distracted by bells and whistles
This is a very common problem. Students tend to focus on the stuff they can see, which can be to your benefit (visual programs are more engaging than command line applications), but like you've discovered, it can also be very distracting.

I think a lot of the problem can be prevented by adding a disclaimer at the top of assignments. Something like this:


  Please note that I am not grading you based on what your program looks like. It can be tempting to obsess over the look and feel, but try to resist that urge! Graphic design is an important field, but this course focuses on [insert focus here]. If you want to explore topics like graphics and design, please do so only after you've finished the main assignment. You also might want to check out [insert graphics course here] after you finish this course.


You might also include guidelines on what's "in scope" for an assignment and what's outside the scope. Something like this:


  Although this assignment includes visual elements, please note that I'm only grading you on your core logic, not what it looks like.
  
  The following are in scope for this assignment:
  
  
  Changes to server-side files.
  Showing the correct components on the client side.
  Getting input from the client to the server.
  
  
  The following are not in scope:
  
  
  The look and feel of the client side.
  CSS changes.
  Layout or style changes.
  
  
  If you're interested in exploring the out-of-scope topics, make sure you do so after completing the in-scope topics.


Or maybe something as simple as this:


  Make sure the basic functionality works before working on the look and feel of your program! Remember: it's better to have an ugly program that works correctly than it is to have a beautiful program that doesn't work.


You might also explain the concept of minimum viable products. It might also be useful to mention feature creep and bike shedding.

A lot of this just comes down to communicating your expectations.