Line an 8"x8" baking pan with aluminum foil; spray foil with non-stick butter-flavored vegetable spray. Toss graham cracker crumbs with brown sugar; press crumb mixture into bottom of pan. Lightly spray with vegetable spray and press again. In a separate bowl, beat cream cheese until softened. Add condensed milk and lime zest; beat until smooth. Add lime juice; beat until well blended. Spread cream cheese mixture in crumb crust. Bake at 350 degrees for 25 to 30 minutes, until center is just set. Cool; cover and chill for at least 2 hours before cutting into bars.