Request for className hook for customizing theme
At present, for using a custom theme, we have to use many complex CSS selectors around flexbox-fix and its accidents div. e.g.

```less
  .flexbox-fix {
      align-items: center;

      div {
        border-radius: 20px !important;

        &:last-child {
          margin-top: 0 !important;
        }
      }

      > div:first-child {
        > div {
          overflow: visible !important;

          &:nth-child(2) {
            margin-top: 10px !important;
          }

          > div {
            > div {
              > div {
                > div {
                  width: 9px !important;
                  height: 9px !important;
                  background-color: transparent !important;
                  border: 2px solid #fff;
                  box-shadow: 0px 0px 2px rgba(0, 0, 0, 0.5);
                }
              }
            }
          }
        }
      }

      &:last-child {
        margin: 10px 0 !important;
        padding: 0 !important;
        border-top: none !important;

        > div {
          width: 20px !important;
          height: 20px !important;
          margin: 0 10px 10px 0 !important;
        }
      }
  }

```

Can we have a more stable, semantic and simple way to do this?