feat(Form): new sass var for preview label font size. fix #2364

Author: bindoon

src/core/utility/_form-element.scss

@@ -45,14 +45,26 @@ $form-element-medium-preview-height: $s-7 !default;
 /// @semantic 大号组件预览态高度
 $form-element-large-preview-height: $s-10 !default;
 
+/// preview label
+/// @semantic 小号label预览态字体
+$form-element-small-preview-label-font-size: $font-size-caption !default;
+
 /// preview text
 /// @semantic 小号组件预览态字体
 $form-element-small-preview-font-size: $font-size-caption !default;
 
+/// preview label
+/// @semantic 中号label预览态字体
+$form-element-medium-preview-label-font-size: $font-size-body-1 !default;
+
 /// preview text
 /// @semantic 中号组件预览态字体
 $form-element-medium-preview-font-size: $font-size-body-1 !default;
 
+/// preview label
+/// @semantic 大号label预览态字体
+$form-element-large-preview-label-font-size: $font-size-subhead !default;
+
 /// preview text
 /// @semantic 大号组件预览态字体
 $form-element-large-preview-font-size: $font-size-subhead !default;

src/form/main.scss

@@ -10,19 +10,19 @@
     &-preview.#{$css-prefix}form-item {
         &.#{$css-prefix}medium {
             #{$form-prefix}-item-label {
-                font-size: $form-element-medium-preview-font-size;
+                font-size: $form-element-medium-preview-label-font-size;
                 line-height: $form-element-medium-preview-height;
             }
         }
         &.#{$css-prefix}small {
             #{$form-prefix}-item-label {
-                font-size: $form-element-small-preview-font-size;
+                font-size: $form-element-small-preview-label-font-size;
                 line-height: $form-element-small-preview-height;
             }
         }
         &.#{$css-prefix}large {
             #{$form-prefix}-item-label {
-                font-size: $form-element-large-preview-font-size;
+                font-size: $form-element-large-preview-label-font-size;
                 line-height: $form-element-large-preview-height;
             }
         }