fix(custom forms): nullify box-shadow for custom forms' states

scss/_variables.scss

@@ -546,15 +546,15 @@ $custom-control-label-disabled-color:           $gray-600 !default;
 $custom-control-indicator-checked-color:        $component-active-color !default;
 $custom-control-indicator-checked-bg:           $component-active-bg !default;
 $custom-control-indicator-checked-disabled-bg:  rgba(theme-color("primary"), .5) !default;
-$custom-control-indicator-checked-box-shadow:   none !default;
+$custom-control-indicator-checked-box-shadow:   null !default;
 $custom-control-indicator-checked-border-color: $custom-control-indicator-checked-bg !default;
 
 $custom-control-indicator-focus-box-shadow:     $input-focus-box-shadow !default;
 $custom-control-indicator-focus-border-color:   $input-focus-border-color !default;
 
 $custom-control-indicator-active-color:         $component-active-color !default;
 $custom-control-indicator-active-bg:            lighten($component-active-bg, 35%) !default;
-$custom-control-indicator-active-box-shadow:    none !default;
+$custom-control-indicator-active-box-shadow:    null !default;
 $custom-control-indicator-active-border-color:  $custom-control-indicator-active-bg !default;
 
 $custom-checkbox-indicator-border-radius:       $border-radius !default;
@@ -563,7 +563,7 @@ $custom-checkbox-indicator-icon-checked:        url("data:image/svg+xml,<svg xml
 $custom-checkbox-indicator-indeterminate-bg:           $component-active-bg !default;
 $custom-checkbox-indicator-indeterminate-color:        $custom-control-indicator-checked-color !default;
 $custom-checkbox-indicator-icon-indeterminate:         url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' width='4' height='4' viewBox='0 0 4 4'><path stroke='#{$custom-checkbox-indicator-indeterminate-color}' d='M0 2h4'/></svg>") !default;
-$custom-checkbox-indicator-indeterminate-box-shadow:   none !default;
+$custom-checkbox-indicator-indeterminate-box-shadow:   null !default;
 $custom-checkbox-indicator-indeterminate-border-color: $custom-checkbox-indicator-indeterminate-bg !default;
 
 $custom-radio-indicator-border-radius:          50% !default;