Fixed theming bug in button and button group.

lib/pixi/button.js

@@ -136,8 +136,8 @@ export default class Button extends PIXI.Container {
                 strokeActive: theme.strokeActive,
                 strokeActiveWidth: theme.strokeActiveWidth,
                 strokeActiveAlpha: theme.strokeActiveAlpha,
-                textStyle: theme.textStyle,
-                textStyleActive: theme.textStyleActive,
+                textStyle: {},
+                textStyleActive: {},
                 style: 'default',
                 radius: theme.radius,
                 disabled: false,
@@ -157,6 +157,9 @@ export default class Button extends PIXI.Container {
 
         this.id = this.opts.id
 
+        this.opts.textStyle = Object.assign({}, theme.textStyle, this.opts.textStyle)
+        this.opts.textStyleActive = Object.assign({}, theme.textStyleActive, this.opts.textStyleActive)
+
         if (typeof this.opts.icon === 'undefined' && typeof this.opts.iconActive !== 'undefined') {
             this.opts.icon = this.opts.iconActive
         } else if (typeof this.opts.icon !== 'undefined' && typeof this.opts.iconActive === 'undefined') {