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.

boss_fight_ui.hpp

@@ -38,7 +38,7 @@ namespace gui {
 
       void init();
       void render(sf::RenderWindow& window);
-      bool mouse(float x, float y);
+      bool mouse(float x, float y, bool hover);
       void bounce_boss(sf::RenderWindow& window);
       bool boss_dead() { return $combat.hp < 0; }
       void configure_sprite();