Here's what I did, had to work around a bunch of default Neospring stuff so this will not work on other sites:

body, button, input {
  --radius: 0;
}

.pillmenu {
  border: none;
  --radius: 0;
}

nav button::before, nav a::before, .card:not(:has(form)) button, .card:not(:has(form)) a.button, .card:not(:has(form)) button:not(dialog *), .card:not(:has(form)) a.button:not(dialog *) {
  border-radius: 0;
}