Fix #1677 disabled button inside basic button group, add global disabled border color

src/definitions/elements/button.less

@@ -211,6 +211,11 @@
   pointer-events: none;
 }
 
+/* Basic Group With Disabled */
+.ui.basic.buttons .ui.disabled.button {
+  border-color: @disabledBorderColor;
+}
+
 /*******************************
              Types
 *******************************/
@@ -714,6 +719,7 @@
 }
 
 
+
 /*--------------
   Labeled Icon
 ---------------*/

src/themes/default/elements/button.variables

@@ -223,8 +223,8 @@
 
 
 /* Basic Group */
-@basicGroupBorder: 1px solid @borderColor;
-@basicGroupBoxShadow: 0px 0px 0px 1px @borderColor;
+@basicGroupBorder: @basicBorderSize solid @borderColor;
+@basicGroupBoxShadow: 0px 0px 0px @basicBorderSize @borderColor;
 
 /*-------------------
       Variations

src/themes/default/globals/site.variables

@@ -351,6 +351,7 @@
 
 @borderColor                   : rgba(39, 41, 43, 0.15);
 @selectedBorderColor           : rgba(39, 41, 43, 0.3);
+@disabledBorderColor           : rgba(39, 41, 43, 0.5);
 
 @solidBorderColor              : #D4D4D5;
 @solidSelectedBorderColor      : #BEBEBF;