I think this is the best I can do for a hover vs. click shader effect. Just do it in a shader based on a uniform setting.

combat_ui.hpp

@@ -17,7 +17,7 @@ namespace gui {
       void render(sf::RenderWindow& window);
       void update_level(GameLevel &level);
       void set_damage(float percent);
-      bool mouse(float x, float y);
+      bool mouse(float x, float y, bool hover);
       void make_button(std::string name, std::wstring label, Events::GUI event);
   };
 }