if you can find it, send what its called in the files and the path to get to it.
There is none, maybe it can be changed with optifine, but I don't know any way for vanilla.
You can change the default font in assets/minecraft/textures/font/ascii.png, which will also affect the level-number. However there is no way to give it a separate font or texture.
The number's green color is hard-coded in vanilla, but can be changed with Optifine's Custom Colors.