qmk_firmware/quantum/rgb_matrix/animations/hue_breathing_anim.h

#ifdef ENABLE_RGB_MATRIX_HUE_BREATHING
RGB_MATRIX_EFFECT(HUE_BREATHING)
#    ifdef RGB_MATRIX_CUSTOM_EFFECT_IMPLS

// Change huedelta to adjust range of hue change. 0-255.
// Hue Breathing - All LED's light up
bool HUE_BREATHING(effect_params_t* params) {
    RGB_MATRIX_USE_LIMITS(led_min, led_max);
    uint8_t  huedelta = 12;
    HSV      hsv      = rgb_matrix_config.hsv;
    uint16_t time     = scale16by8(g_rgb_timer, rgb_matrix_config.speed / 8);
    hsv.h             = hsv.h + scale8(abs8(sin8(time) - 128) * 2, huedelta);
    RGB rgb           = hsv_to_rgb(hsv);
    for (uint8_t i = led_min; i < led_max; i++) {
        RGB_MATRIX_TEST_LED_FLAGS();
        rgb_matrix_set_color(i, rgb.r, rgb.g, rgb.b);
    }
    return rgb_matrix_check_finished_leds(led_max);
}

#    endif  // RGB_MATRIX_CUSTOM_EFFECT_IMPLS
#endif      // DISABLE_RGB_HUE_BREATHING