fix(material): use null value for gradients

packages/material/scss/_variables.scss

@@ -245,7 +245,7 @@ $base-bg: #fff !default;
 
 $base-border: rgba( black, .08 ) !default;
 /// The gradient background of the components' chrome area.
-$base-gradient: $base-bg, darken( $base-bg, 2% ) !default;
+$base-gradient: null !default;
 
 /// The text color of hovered items.
 $hovered-text: $base-text !default;
@@ -254,7 +254,7 @@ $hovered-bg: rgba( black, .07) !default;
 /// The border color of hovered items.
 $hovered-border: rgba( black, .15 ) !default;
 /// The gradient background of hovered items.
-$hovered-gradient: $hovered-bg, darken( $hovered-bg, 2% ) !default;
+$hovered-gradient: null !default;
 
 /// The text color of selected items.
 $selected-text: $accent-contrast !default;
@@ -263,7 +263,7 @@ $selected-bg: $accent !default;
 /// The border color of selected items.
 $selected-border: rgba( black, .1 ) !default;
 /// The gradient background of selected items.
-$selected-gradient: none !default;
+$selected-gradient: null !default;
 
 $selected-hover-color: $selected-text !default;
 $selected-hover-bg: mix( $selected-bg, black, 92) !default;
@@ -557,7 +557,7 @@ $grid-border: rgba( black, .12 ) !default;
 $grid-header-bg: $header-bg !default;
 $grid-header-text: rgba(0, 0, 0, .54) !default;
 $grid-header-border: $grid-border !default;
-$grid-header-gradient: $header-gradient !default;
+$grid-header-gradient: null !default;
 
 $grid-alt-bg: transparent !default;
 $grid-alt-text: $grid-text !default;
@@ -820,7 +820,7 @@ $calendar-border: $widget-border !default;
 $calendar-header-bg: $header-bg !default;
 $calendar-header-text: $header-text !default;
 $calendar-header-border: $header-border !default;
-$calendar-header-gradient: $header-gradient !default;
+$calendar-header-gradient: null !default;
 $calendar-header-shadow: none !default;
 
 $calendar-header-cell-text: $calendar-header-text !default;