Tone and mood are often confusing terms. Tone is the author's attitude toward the work; we can used words like positive, negative, sarcastic, critical, and sympathetic, for example,to describe tone.
Mood, on the other hand, pertains to the emotional atmosphere in the work. Sad, happy, melancholy, and delighted are words that can describe mood.
The exact setting for this Thomas poem is unknown except that the speaker is at the bedside of his father who is dying. The tone (often referred to as voice in a poem) is urgent; he wants his father to fight against death, to "rage against the dying of the light." As he sits next to his dying father whom he loves, his mood is a mixture of despair ("What if he should actually die now?" he may ask himself) and anger ("How can this be happening to my father?" he may question.).