message: feat(Button): support CSS vars gradient background color
diff: left: 50%;
  width: 100%;
  height: 100%;
  - background-color: var(--van-black);
  + background: var(--van-black);
  border: inherit;
  border-color: var(--van-black);
  border-radius: inherit; /* inherit parent's border radius */
  &--default {
  color: var(--van-button-default-color);
  - background-color: var(--van-button-default-background-color);
  + background: var(--van-button-default-background-color);
  border: var(--van-button-border-width) solid
  var(--van-button-default-border-color);
  }
  &--primary {
  color: var(--van-button-primary-color);
  - background-color: var(--van-button-primary-background-color);
  + background: var(--van-button-primary-background-color);
  border: var(--van-button-border-width) solid
  var(--van-button-primary-border-color);
  }
  &--success {
  color: var(--van-button-success-color);
  - background-color: var(--van-button-success-background-color);
  + background: var(--van-button-success-background-color);
  border: var(--van-button-border-width) solid
  var(--van-button-success-border-color);
  }
  &--danger {
  color: var(--van-button-danger-color);
  - background-color: var(--van-button-danger-background-color);
  + background: var(--van-button-danger-background-color);
  border: var(--van-button-border-width) solid
  var(--van-button-danger-border-color);
  }
  &--warning {
  color: var(--van-button-warning-color);
  - background-color: var(--van-button-warning-background-color);
  + background: var(--van-button-warning-background-color);
  border: var(--van-button-border-width) solid
  var(--van-button-warning-border-color);
  }
  &--plain {
  - background-color: var(--van-button-plain-background-color);
  + background: var(--van-button-plain-background-color);
  &.van-button--primary {
  color: var(--van-button-primary-background-color);
  