border-bottom-color

border-bottom-color: red;

border-bottom-color: #32a1ce;

border-bottom-color: rgb(170 50 220 / 0.6);

border-bottom-color: hsl(60 90% 50% / 0.8);

border-bottom-color: transparent;

<section class="default-example" id="default-example">
  <div class="transition-all" id="example-element">
    This is a box with a border around it.
  </div>
</section>

#example-element {
  background-color: #eeeeee;
  color: black;
  border: 0.75em solid;
  padding: 0.75em;
  width: 80%;
  height: 100px;
}

/* <color> values */
border-bottom-color: red;
border-bottom-color: #ffbb00;
border-bottom-color: rgb(255 0 0);
border-bottom-color: hsl(100deg 50% 25% / 75%);
border-bottom-color: currentColor;
border-bottom-color: transparent;
/* Global values */
border-bottom-color: inherit;
border-bottom-color: initial;
border-bottom-color: revert;
border-bottom-color: revert-layer;
border-bottom-color: unset;

<div class="my-box">
  <p>
    This is a box with a border around it. Note which side of the box is
    <span class="red-text">red</span>.
  </p>
</div>

.my-box {
  border: solid 0.3em gold;
  border-bottom-color: red;
  width: auto;
}
.red-text {
  color: red;
}