qmk_firmware_custom/keyboards/nano/keymaps/default/keymap.c

#include QMK_KEYBOARD_H

#define _MAIN 0
#define _FN 1

#define _______ KC_TRNS
#define KC_X0 LT(_FN, KC_ESC)

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
  [_MAIN] = LAYOUT(
     KC_VOLU, KC_MPLY, KC_MPRV, KC_PGUP,
     KC_VOLD, KC_MUTE, KC_MNXT, KC_PGDN
  ),

  [_FN] = LAYOUT(
     KC_F,    _______, RGB_HUI, _______,
     RGB_TOG, RGB_MOD, RGB_HUD, _______
  )
};