docs/_static/css/lang-switcher.css

.lang-switcher {
  display: flex;
  align-items: center;
  justify-content: center;
}

.lang-switcher__select {
  appearance: none;
  -webkit-appearance: none;
  -moz-appearance: none;
  padding: 6px 28px 6px 10px;
  border-radius: 999px;
  border: 1px solid rgba(0, 0, 0, 0.18);
  background-color: #ffffff;
  color: #333333;
  font-size: 13px;
  line-height: 18px;
  box-shadow: 0 1px 2px rgba(0, 0, 0, 0.08);
  cursor: pointer;
  background-image: linear-gradient(45deg, transparent 50%, #667085 50%),
    linear-gradient(135deg, #667085 50%, transparent 50%);
  background-position: calc(100% - 16px) 50%, calc(100% - 11px) 50%;
  background-size: 5px 5px, 5px 5px;
  background-repeat: no-repeat;
}

.lang-switcher__select:focus {
  outline: none;
  border-color: rgba(41, 128, 185, 0.65);
  box-shadow: 0 0 0 3px rgba(41, 128, 185, 0.18);
}

.wy-side-nav-search .lang-switcher {
  margin-top: 10px;
}

.wy-side-nav-search .lang-switcher__select {
  border-color: rgba(255, 255, 255, 0.18);
  background-color: rgba(255, 255, 255, 0.08);
  color: #ffffff;
  box-shadow: none;
  background-image: linear-gradient(45deg, transparent 50%, rgba(255, 255, 255, 0.75) 50%),
    linear-gradient(135deg, rgba(255, 255, 255, 0.75) 50%, transparent 50%);
}

.wy-side-nav-search .lang-switcher__select:focus {
  border-color: rgba(255, 255, 255, 0.45);
  box-shadow: 0 0 0 3px rgba(255, 255, 255, 0.12);
}