Hello, if you wan't to make the game looks like anime/mango, you have two possibilities :
-> You can change all textures of the game with a image editor (I remember that i've heard about a mass image editor : using the same filter automaticaly on a great number of images)
-> or you can change the game rendering engine (good luck
) but this way is very complicated I thing, but if you can get through it, you could add a cell-shading in the rendering engine and this way, you wouldn't have to change any textures.
Hope you good luck