If you want a true outline, you need to do that in the texture itself. Create a bitmap font using the Font Maker, and don't add it to an atlas (just make it a stand-alone font/texture). Select the font prefab, give it some padding by hitting +1 Transparent Border a few times. Now open the texture in Photoshop and add a border to it. Save, and it'll look as you want it. You can now add it to an atlas if you like.
