Example for qmk_firmware to turn rgb underglow off after a period of inactivity

keymap.c

#ifdef RGBLIGHT_ENABLE
#define IDLE_TIMEOUT 600000   // Blank underglow after 600 secs of inactivity
extern rgblight_config_t rgblight_config;
static uint32_t idle_timer;
static bool idle_timeout_enabled;
void keyboard_post_init_user(void) {
    idle_timer = timer_read32();
    idle_timeout_enabled = rgblight_config.enable ? true : false;
    xprintf("idle_timeout init: idle_timeout_enabled = %u\n", idle_timeout_enabled);
}
void matrix_scan_user(void) {
    if (idle_timeout_enabled) {
        bool shouldenable = timer_elapsed32(idle_timer) < IDLE_TIMEOUT ? true : false;
        if (shouldenable != rgblight_config.enable) {
            xprintf("idle_timeout override: toggling\n");
            // Toggle the underglow. Unfortunately for split keyboards we need
            // to use the eeprom version in order to have it propagate across halves
#ifdef SPLIT_KEYBOARD
            rgblight_toggle();
            RGB_DIRTY = true;
#else
            rgblight_toggle_noeeprom();
#endif
        }
    }
}
bool process_record_user(uint16_t keycode, keyrecord_t *record) {
    if (record->event.pressed) {
        idle_timer = timer_read32();
    }
    switch (keycode) {
    case RGB_TOG: // Monitor for change in global status
        // Split keyboards need to trigger on key-up for edge-case issue
        // See corresponding case in quantum.c
#ifndef SPLIT_KEYBOARD
        if (record->event.pressed) {
#else
        if (!record->event.pressed) {
#endif
            // First restore rgblight enabled status to original
            rgblight_config.enable = idle_timeout_enabled;
            idle_timeout_enabled = idle_timeout_enabled ? false : true;
            xprintf("idle_timeout enable: idle_timeout_enabled = %u, rgblight_config.enabled = %u\n", idle_timeout_enabled, rgblight_config.enable);
        }
        return true; // Let main quantum code handle the actual toggling
    }
    return true;
}
#endif