Added textAlpha and textActiveAlpha to Button and ButtonGroup.

lib/pixi/button.js

@@ -81,6 +81,8 @@ export default class Button extends PIXI.Container {
      *     for possible options.
      * @param {number} [opts.textStyleActive=Theme.textStyleActive] - A textstyle object for the styling of the label when the
      *     button is activated. See PIXI.TextStyle for possible options.
+     * @param {number} [opts.textAlpha=Theme.textAlpha] - The alpha value of the text.
+     * @param {number} [opts.textActiveAlpha=Theme.textActiveAlpha] - The alpha value of the text when activated.
      * @param {string} [opts.style=default] - A shortcut for styling options. Possible values are default, link.
      * @param {number} [opts.radius=Theme.radius] - The radius of the four corners of the button (which is a rounded rectangle).
      * @param {boolean} [opts.disabled=false] - Is the button disabled? When disabled, the button has a lower alpha value
@@ -138,6 +140,8 @@ export default class Button extends PIXI.Container {
                 strokeActiveAlpha: theme.strokeActiveAlpha,
                 textStyle: {},
                 textStyleActive: {},
+                textAlpha: theme.textAlpha,
+                textActiveAlpha: theme.textActiveAlpha,
                 style: 'default',
                 radius: theme.radius,
                 disabled: false,
@@ -566,10 +570,12 @@ export default class Button extends PIXI.Container {
         if (this._active) {
             if (this.text) {
                 this.text.style = this.opts.textStyleActive
+                this.text.alpha = this.opts.textActiveAlpha
             }
         } else {
             if (this.text) {
                 this.text.style = this.opts.textStyle
+                this.text.alpha = this.opts.textAlpha
             }
         }