There has been a bug in minecraft for a very long time that causes signs to not render properly on mac.
https://bugs.mojang.com/browse/MC-1313 is the link to this issue. This mod is a simple way to fix these signs. You can install this mod on Windows/Linux, but there is no point because this is a mac-only issue.
If you do not want to use this json, make sure to delete the "downloads" portion of the json, or minecraft will just troll you by replacing your jar with a vanilla jar. If you do this, make sure your brackets are balanced.