You really should research mipmapping. It's exactly the problem here. Proper mipmapping or mipmapping based anisotropic filtering provides a stepless-looking level of detail into the depth. Without it you have aliasing in the background. With only two levels of mipmapping you have a border between highres and lowres textures. SE implemented their own technique it seems. They only have background and foreground. That's okay for objects if you can't use tesselation, but for textures it looks really bad.