[css] SiYuan 按键彩虹渐变效果

实现 SiYuan 按键的彩虹渐变效果，参考了 https://siyuannote.com/docs/article/1724628228

更新：适配写味主题，优化文字颜色

提醒：可以自行调节按钮大小，第二行 width 是宽度，第三行 height 是高度

#barWorkspace {
  width: 80px;
  height: 25px;
  border-radius: 12px !important;
  background: linear-gradient(90deg, #FF6B6B, #FF7757, #FFAB3D, #6BF178, #6DD3FF, #D7B5FE, #FFB5E1, #FF6B6B);
  background-size: 1600% 100%;
  animation: rainbowButton 12s linear infinite;
  user-select: none;
  transition: filter 0.3s ease;
  display: flex;
  align-items: center;
  justify-content: center;
}
#barWorkspace span.toolbar__text {
  font-size: 12px !important;
  font-weight: 600 !important;
  color: white !important;
  text-transform: none !important;
}
#barWorkspace svg.toolbar__svg {
  color: white !important;
  opacity: 1 !important;
  margin-top: 2.5px !important;
}
#barWorkspace:hover { filter: brightness(0.8); }
#barWorkspace:active { filter: brightness(0.7); }
@keyframes rainbowButton {
  0% { background-position: 0% 0%; }
  100% { background-position: -200% 0%; }
}