I may have found a solution. The culprit seems to be this line in Neospring's base CSS:

#response\:25fbe6e3ac0a48c965b6e47f28e857770915e8890e640cebd5be4098f35d655a .response_body:not(.response_reply *) {
  border-radius: var(--radius) !important;
  border-top: solid 1px var(--color-super-lowered) !important;
}

Since it's !important you clearly aren't supposed to override it, and I don't think you even can, but you can move the code for your border to another element.

Everything seems to look the same if you replace .card with .card-nest in this list of elements:

.card, .cover.round.shadow, textarea, nav, .convertible.pillmenu, div.nav_side.gap-1.flex .dropdown .shadow-md.inner, #warning_dialog, #status\:0bd7e624-07e2-493a-b3a9-f54e7ea99ebd, pre {
  border-radius: 0px!important;
  border-width:16px!important;
  border-style:solid!important;
  border-image: url("/api/util/ext/image?img=https://file.garden/ZsYPtSRhGGFNpDxv/vincent%20layout/vin%20border%201.png") 17 fill round!important;
}