MText Background Color (sysvar or DXF #)

Two questions:

Is there a sysvar for the MText Background??

If not, how can I enable the background (programmatically) from this code
snippet??
I know I'm doing something wrong, but I can't put my finger on it.

Code:

(defun C:BGTest ( / pt1 pt2 eLast ent2 strString strNewString)
(command "textstyle" tStyle (initdia) "mtext" (setq pt1 (getpoint "\n
Pick first corner of MText box..."))
(setq pt2 (getcorner pt1 "\n Pick second corner of MText box...")))
(setq eLast (entlast))
(setq ent2 (entget eLast))
(setq strString (cdr (assoc 1 ent2)))
(setq strNewString (subst (cons 90 0) (assoc 90 ent2) ent2)) ;; Is it DXF
90 for the background??!?
(entmod strNewString)
(princ)
)

Thanks in advance!

 

From Lee Ambroious' MTEXTMASK:

(progn
(cond ((= kWordBackgroundMask "None")
(setq mask_list (list (cons 90 2)(cons 63 0)(cons 45 dOffset)(cons 441 0))
dxf90 2
dxf63 nil
dxf45 nil
dxf441 nil
)
)
((and (= kWordBackgroundMask "Color")(/= (assoc 420 maskClr) nil))
(setq mask_list (list (cons 90 1)(cons 63 1)(cons 421 (cdr (assoc 420
maskClr)))(cons 45 dOffset)(cons 441 (cdr (assoc 420 maskClr))))
dxf90 1
dxf63 2
dxf45 dOffset
dxf441 (cdr (assoc 420 maskClr))
)
)
((and (= kWordBackgroundMask "Color")(= (assoc 420 maskClr) nil))
(setq mask_list (list (cons 90 1)(cons 63 (cdr (assoc 62 maskClr)))(cons 45
dOffset)(cons 441 2146608))
dxf90 1
dxf63 (cdr (assoc 62 maskClr))
dxf45 dOffset
dxf441 nil
)
)
((= kWordBackgroundMask "Background")
(setq mask_list (list (cons 90 3)(cons 63 3)(cons 45 dOffset)(cons 441 0))
dxf90 3
dxf63 256
dxf45 dOffset
dxf441 0
)
)
)