Are you sure it was in "project settings" and not "Editor Settings"? I am going to assume it was editor settings, because you are writing about "code editor" and its appearance to my knowledge is not configurable in project settings.
If by template you mean theme preset then it can be changed back in
Editor Settings... ->
You can revert changes of code editor one-by-one in
Editor Settings... and there browse through items in
Text Editor (on the left), and every time you see a revert icon ⟲ (in right part of window) click it.
Faster option would be to delete your editor configuration file (e.g. on Linux it's at
~/.config/godot/editor_settings-3.tres). But I have to stress that you will lose ALL editor settings (e.g. recent projects), not just code editor settings.