remember there are two separate gallery layouts there A and B (make sure you edit the correct one you are using)
when you define an image for the gallery
image img3 = ("images/gallery/gallery3.jpg")
you must show img3 not gallery3 before it is it unlocked in the galler, you can:
show img3
or
scene img3
the background for those buttons are in the code for the button
if gallery_page > 0:
textbutton "{color=#000}Previous{/color}":
action SetVariable("gallery_page", gallery_page - 1)
xalign 0.3
yalign 0.98
background "#fff8" #<<--- it is what it is
fff is white but with an 8 in the alpha channel make it semi-transparent (looks light grey)