τόνος is the correct spelling.
So is όνομά σου - you keep the original tonos on όνομα when you add the extra tonos. You add the extra tonos when a noun with tonos on the third-to-last syllable is followed by a possessive pronoun.
I guess this is because the two words are pronounced as one unit, and there has to be tonos somewhere in the last three syllables of the unit.