2色のボーダー

h2 {
  position: relative;
  width: 250px;
  margin: 20px auto;
  padding: 10px;
  color: #ffffff;
  border-bottom: 3px solid #007db9;
}
    
h2:before {
  content: "";
  position: absolute;
  left: 0;
  bottom: -3px;
  width: 70px;
  height: inherit;
  border-bottom: 3px solid #ffffff;
}
https://into-the-program.com/two-color-border/