Make new textures fit existing style

mods/default/README.txt

mods/default/textures/default_cobble.png

mods/default/textures/default_dirt.png

mods/default/textures/default_furnace_bottom.png

mods/default/textures/default_furnace_front.png

mods/default/textures/default_furnace_front_active.png

mods/default/textures/default_furnace_side.png

mods/default/textures/default_furnace_top.png

mods/default/textures/default_grass.png

mods/default/textures/default_mossycobble.png