Add additional background variables to button

src/definitions/elements/button.less

@@ -33,7 +33,7 @@
   outline: none;
   border: none;
   vertical-align: @verticalAlign;
-  background-color: @backgroundColor;
+  background: @background;
   color: @textColor;
 
   font-family: @fontFamily;
@@ -49,7 +49,6 @@
   text-align: center;
   text-decoration: none;
 
-  background-image: @backgroundImage;
   border-radius: @borderRadius;
   box-shadow: @boxShadow;
 
@@ -644,7 +643,6 @@
 .ui.basic.buttons .button,
 .ui.basic.button {
   background: @basicBackground !important;
-  background-image: none;
   color: @basicTextColor !important;
   font-weight: @basicFontWeight;
   border-radius: @basicBorderRadius;

src/themes/default/elements/button.variables

@@ -12,6 +12,7 @@
 /* Style */
 @backgroundColor: #E0E1E2;
 @backgroundImage: none;
+@background: @backgroundColor @backgroundImage;
 @verticalPadding: 0.7857142em;
 @horizontalPadding: 1.5em;
 
@@ -182,7 +183,7 @@
 @basicTextColor: @textColor;
 @basicColoredBorderSize: 2px;
 
-@basicBackground: transparent;
+@basicBackground: transparent none;
 @basicFontWeight: normal;
 @basicBorder: 1px solid @borderColor;
 @basicBoxShadow: 0px 0px 0px @basicBorderSize @borderColor inset;