I don't think theft is (morally) based on consent because consent is a function of preference. If I consent to some action then the consent is the communication of my preference.
Consider that I'm unconscious and dying and a passing Dr needs to cut a hole in my neck with a penknife for me to live. I can't give consent, therefore it is assault? But it goes that it is assumed that if I could give consent to the Dr I would, presumably because the consent communicates my preference to live.
My point is that the act of taxation, taken in isolation from the frame of reference of someone preferring the end result of said taxation, which is the say the benefits of taxation, can't be called theft because taxation for that person is preferable and theft, by definition, is (universally) not preferable.
This is why a lot of people can't accept that tax is theft, because they experience taxation as preferable to non taxation.