<button>Hello World</button>

body {
  padding: 20px;
  --background-color: white;
  background-color: var(--background-color);
}
button {
  width: max-content;
  padding: 0.5em 1em;
  border: none;
  outline: none;
  border-radius: 999px;
  background-color: var(--background-color);
  font-size: 30px;
  font-weight: 700;
  color: navy;
  position: relative;
  display: flex;
  justify-content: center;
  align-items: center;
  cursor: pointer;
}
button::after {
  --border-width: 10px;
  content: "";
  width: calc(100% + var(--border-width));
  height: calc(100% + var(--border-width));
  border-radius: 999px;
  background-image: linear-gradient(to bottom right, red, blue);
  position: absolute;
  z-index: -1;
}