Γεια σου Ιάκοβε.
The word θέα is pronounced as thea.
The word θεία is pronounced as thia.
The adjective θείος, θεία, θείο means devine and it comes from the ancient Greek. It has the same origin with the word θείος οr θεία (aunt or uncle). Actually it is the same word with a different meaning. Their origin is the word θεός (= god). Tonos is not put only in the case that the word is written with capital letters. Example ΘΕΙΟΣ ΘΕΙΑ ΘΕΙΟ.
The word θέα comes from the ancient word θεώμαι = (I see, I watch).Θέατρο (=theatre) has also the same origin.
As for the meaning of words which sound the same or have the same spelling, although they are different, we guess the meaning from the context (from the surrounding words.)