Bake the white cake in a 13x9-inch pan according to package directions and let it cool for about 20 minutes
Poke holes across the warm cake with a fork almost to the bottom of the pan
Dissolve the strawberry gelatin in boiling water, stir in cold water, then slowly pour over the cake
Refrigerate the cake for at least 3 hours until chilled and set
Whisk the pudding mix with cold milk until smooth, then fold in the whipped topping
Spread the pudding mixture evenly over the chilled cake
Arrange blueberries and sliced strawberries on top to create a flag pattern
Keep refrigerated until ready to serve