In keymap_poker.c:
const uint8_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
    KEYMAP(ESC,     1,   2,   3,   4,   5,   6,   7,   8,   9,   0,   MINS,EQL,BSPC, FN0, \
        TAB,    Q,   W,   E,   R,   T,   Y,   U,   I,   O,   P,   LBRC,RBRC,BSLS, MY_COMPUTER, \
        CAPS,   A,   S,   D,   F,   G,   H,   J,   K,   L,   SCLN,QUOT, ENTER, CALCULATOR,  \
        LSFT,   Z,   X,   C,   V,   B,   N,   M,   COMM,DOT, SLSH,RSFT,UP,PAUS, \
        LCTL,   LGUI,LALT,          SPC,                     FN0, APP,LEFT,DOWN,RIGHT),
    /* 1: FN0 */
    KEYMAP(GRAVE,   F1,     F2,     F3,     F4,     F5,     F6,     F7,     F8,     F9,     F10,    F11,    F12,    TRNS,   FN1,\
        TRNS,   TRNS,   TRNS,   TRNS,   TRNS,   TRNS,   TRNS,   PSCREEN,SCROLLLOCK, TRNS,   TRNS,   TRNS,   TRNS,   TRNS,   FN2,\
        TRNS,   TRNS,   TRNS,   TRNS,   TRNS,   TRNS,   TRNS,   INSERT, HOME,       PGUP,   TRNS,   TRNS,   TRNS,   FN3,\
        TRNS,   TRNS,   TRNS,   TRNS,   TRNS,   TRNS,   TRNS,   DEL,    END,        PGDOWN, TRNS,   TRNS,   FN5,    FN4,\
        TRNS,   TRNS,   TRNS,                           TRNS,                           TRNS,   FN7,    TRNS,   FN6,    TRNS)


};
const uint16_t PROGMEM fn_actions[] = {
    [0] = ACTION_LAYER_MOMENTARY(1),
    [1] = ACTION_FUNCTION(BACKLIGHT_MODE1),
    [2] = ACTION_FUNCTION(BACKLIGHT_MODE2),
    [3] = ACTION_FUNCTION(BACKLIGHT_MODE3),
    [4] = ACTION_FUNCTION(BACKLIGHT_MODE4),
    [5] = ACTION_FUNCTION(BACKLIGHT_INCREASE),
    [6] = ACTION_FUNCTION(BACKLIGHT_DECREASE),
    [7] = ACTION_FUNCTION(BACKLIGHT_OFF)
};

void action_function(keyrecord_t *record, uint8_t id, uint8_t opt)
{
    switch (id)
    {
        case BACKLIGHT_MODE1:
        showColor(253,102,0); //Portal orange
        break;
        case BACKLIGHT_MODE2:
        showColor(0,120,255); //Portal blue
        break;
        case BACKLIGHT_MODE3:
        showColor(255,255,25);
        break;
        case BACKLIGHT_MODE4:
        showColor(rand()%255,rand()%255,rand()%255));
        break;
        case BACKLIGHT_INCREASE:
        increase();
        break;
        case BACKLIGHT_DECREASE:
        decrease();
        break;
        case BACKLIGHT_OFF:
        showColor(0,0,0);
        break;
        default:
        showColor(0,0,0);
        break;
    }
}

------------------------------
in backlight.c:
#define BACKLIGHT_MODE1 1
#define BACKLIGHT_MODE2 2
#define BACKLIGHT_MODE3 3
#define BACKLIGHT_MODE4 4
#define BACKLIGHT_INCREASE 5
#define BACKLIGHT_DECREASE 6
#define BACKLIGHT_OFF 7

void decrease(void);
void increase(void);
---------------------------
BACKLIGHT_LEVELS defined as 1
error is:
obj_gh60_lufa/common/backlight.o: In function `backlight_init':
D:\Documents\projects\CherryMX\firmware\portalboard\keyboard\gh60/../../common/backlight.c:31: undefined reference to `backlight_set'
obj_gh60_lufa/common/backlight.o: In function `backlight_increase':
D:\Documents\projects\CherryMX\firmware\portalboard\keyboard\gh60/../../common/backlight.c:43: undefined reference to `backlight_set'
obj_gh60_lufa/common/backlight.o: In function `backlight_decrease':
D:\Documents\projects\CherryMX\firmware\portalboard\keyboard\gh60/../../common/backlight.c:55: undefined reference to `backlight_set'
obj_gh60_lufa/common/backlight.o: In function `backlight_toggle':
D:\Documents\projects\CherryMX\firmware\portalboard\keyboard\gh60/../../common/backlight.c:63: undefined reference to `backlight_set'
obj_gh60_lufa/common/backlight.o: In function `backlight_step':
D:\Documents\projects\CherryMX\firmware\portalboard\keyboard\gh60/../../common/backlight.c:76: undefined reference to `backlight_set'
obj_gh60_lufa/common/backlight.o:D:\Documents\projects\CherryMX\firmware\portalboard\keyboard\gh60/../../common/backlight.c:84: more undefined references to `backlight_set' follow