How can I correctly add a shadow and a gradient to my triangular shape?

Question

I want to make the following design:

I tried with :after and :before but it does not work. Here’s my current code:

Answer 1

Change to (only changed lines listed, keep everything else as-is):

.design:before {
  ...
  border-left: 114px solid transparent;
  border-right: 114px solid transparent;
  ...
  width: 0;
}