Resolve "Text Color of Button is too dark"

Closes #2 (closed)

invenio_theme_tuw/assets/semantic-ui/less/invenio_theme_tuw/theme/elements/button.variables

@@ -11,6 +11,7 @@
 
 /* Text */
 @borderRadius: 0px;
+@textColor: @white;
 
 /* Icon */
 @iconColor: @white;
@@ -27,4 +28,4 @@
 --------------------*/
 
 /* Basic */
-@basicBorderRadius: @borderRadius;
\ No newline at end of file
+@basicBorderRadius: @borderRadius;