In what year did the Girl Scout cookie, Thin MInt, change from off-white to dark brown in color in the center?

That really depends. At time GSUSA had dozens of authorized bakers, and they all had slightly different recipes.



I know that I quit buying Thin Mints in the early 1980's when I got some that were white on the inside and pretty flavorless. That was different from the ones I ate as a child, with chocolate insides (late 1960's to early 1970's).



In 1985 I started graduate school, and bought chocolate-inside cookies from the daughter of one of the staff. Much better!