edit the gallery.rpy file and find this section
for i in range(start, end + 1):
hbox:
spacing maxthumbx - 20
xalign 0.5
yalign 0.1
text gallery_items[i].name # we will be editing this line
and make this change
text gallery_items[i].name # we will be editing this line: # add a colon
outlines [ (1, "#e9ec09", 0, 0) ] # adjust the outlines as required