update_lights.c

void update_lights(bool beat_detected) {                                        
        static uint8_t blueness = 0;                                            
                                                                                
        if (beat_detected) {                                                    
                blueness = MAX_BRIGHTNESS;                                      
        } else if (blueness >= BRIGHTNESS_DECAY) {                              
                blueness = blueness - BRIGHTNESS_DECAY;                         
        } else {                                                                
                blueness = 0;                                                   
        }                                                                       
        APA102_Fill(led_strip, APA102_CreateFrame(STRIP_BRIGHTNESS, 0, 0, blueness));
}