gitea/web_src/css/modules/select.css

.gitea-select {
  position: relative;
}

.gitea-select select {
  appearance: none; /* hide default triangle */
}

/* ::before and ::after pseudo elements don't work on select elements,
   so we need to put it on the parent. */
.gitea-select::after {
  position: absolute;
  top: 12px;
  right: 8px;
  pointer-events: none;
  content: "";
  width: 14px;
  height: 14px;
  mask-size: cover;
  -webkit-mask-size: cover;
  mask-image: var(--octicon-chevron-right);
  -webkit-mask-image: var(--octicon-chevron-right);
  transform: rotate(90deg); /* point the chevron down */
  background: currentcolor;
}