Modify the keyboard opt symbols Is there any way to (or existing program which can) modify the keyboard symbols you get when holding down the option key?

I.e. I want to be able to modify these mappings:
If you want to do this without using external application there is a good answer here

https://apple.stackexchange.com/a/130099/46541

DefaultKeyBinding.dict

Create ~/Library/KeyBindings/DefaultKeyBinding.dict and enter

{ "~l" = (insertText:, "@"); }


This rebinds ⌥L to insert @

~ = ⌥


Quit and reopen applications for the change to take effect

Some examples of external sources

https://apple.stackexchange.com/a/112381/46541