It is not only a question of harshness, but of meaning.
Even though you can find dictionary entries that will say (CALD)
to take something without the permission or knowledge of the owner and keep it
where it seems that it does not matter how you took it (intentionally or not), in actual use "to steal" implies intention as per following definition (MW)
to take the property of another wrongfully and especially as a habitual or regular practice
To say wrongfully implies "bad" intention and does not convey the meaning that you have in mind without further explanation.
I don't know of the word which does; all related words I looked at have this implication except the most basic and widest term: "to take".