Why isn't the theme selection working?

Question

In my calculator code, I added a theme selection. I am using the syntax from stack overflow (selectedIndex) but I can't get the form to validate. The code only works for selectedIndex(0) but not the rest. Can anyone fix this (I am using Jquery)
https://code.sololearn.com/W3y6iXfnxw7f/?ref=app

Answer 1

Did you ever find a solution to this?