siyuan/app/src/assets/scss/component/_select.scss

.b3-select {
  border: 0;
  border-radius: 4px;
  padding: 4px 8px;
  line-height: 20px;
  box-sizing: border-box;
  color: var(--b3-theme-on-background);
  transition: box-shadow 120ms 0ms cubic-bezier(0, 0, 0.2, 1);
  background-color: var(--b3-theme-background);
  min-width: 200px;
  height: 28px;
  font-size: 14px;
  overflow: hidden;

  &:focus {
    box-shadow: 0 0 0 3px var(--b3-theme-primary-lighter);
  }
}