There is a lot of anger in this poem. As opposed to fear of death, I would say the theme is anger at death. Consider the first stanza:
Do not go gentle into that good night,
Old age should burn and rage at close of day;
Rage, rage against the dying of the light. (lines 1-03)
In this first stanza, the word “rage” is used three times! The speaker is definitely angry.
Both simile and metaphor are types of figurative language that make comparisons. A simile compares things but qualifies the comparison by saying something is like something else, whereas a metaphor describes something as something else. The entire poem is a metaphor, because one cannot really rage against death. However, the speaker is really telling us to enjoy life and make the most of it, and not go quietly. Consider line 4:
Though wise men at their end know dark is right
In this line, the word “dark” is a metaphor for death. This helps to add to the theme of anger by describing death as darkness. Our theme follows from the metaphor.
The poet also throws in a simile to make his point.
Blind eyes could blaze like meteors and be gay (line 14)
He does not say the blind eyes are meteors directly, he compares them to meteors using a simile with the words “like meteors” in order to add to the concept of blazing eyes, which is a sign of anger.