You need a small image that's formatted as a 3x3 grid, with the squares in the corners being the corners of the border, and the squares on the sides being the repeating pattern of the sides of the border.

Then you can use code like this (which I copied from @bachira's page):

.card-nest.w-full.shadow {
  border-radius: 25px;
  border-width: 12px !important;
  border-style: solid !important;
  border-image: url(INSERT IMAGE URL HERE) 17 fill round !important;
}