Michael Großklaus

Without rounded corners:

border: solid 0.25rem;
border-image: linear-gradient(to bottom right, darkmagenta, aquamarine);

With rounded corners:

border: solid 0.25rem transparent;
background:
  linear-gradient(white, white) padding-box, 
  linear-gradient(to bottom right, darkmagenta, aquamarine) border-box;

Drawback: You need to set a solid color as background, so it cannot be transparent.