Added style class definitions to *Style.d.ts

components/lib/button/style/ButtonStyle.d.ts

@@ -1,3 +1,19 @@
+/**
+ *
+ * Button is an extension to standard button element with icons and theming.
+ *
+ * [Live Demo](https://www.primevue.org/button/)
+ *
+ * @module buttonstyle
+ *
+ */
 import { BaseStyle } from '../../base/style';
 
+export enum ButtonClasses {
+    root = 'p-button',
+    loadingIcon = 'p-button-loading-icon',
+    icon = 'p-button-icon',
+    label = 'p-button-label'
+}
+
 export interface ButtonStyle extends BaseStyle {}