What is the theme of the poem "Death be not Proud" by John Donne?

The central theme of the poem "Death be not Proud" by John Donne is the powerlessness of death. According to Donne, death is but a pathway to eternal life, and as such is not something "mighty...

Donne boldly addresses death and speaks to him in dismissive terms. In the opening four lines, however, Donne offers no evidence to support his initial assertion that Death should not be proud; evidence isn’t really given until line 5, and even in lines 5–8 we get very little supporting evidence. Not until the sestet do we get a list of reasons: Death is the slave of "Fate, Chance, kings, and desperate men"; Death dwells with unsavory people ("Poison, War, and Sickness"); "poppy or charms can make us sleep as well."

And then, picking up the word "sleep" from line 12, Donne goes on to contrast the "short sleep" of Death (13) with our eternal awakening. He thus ends triumphantly, "Death, thou shalt die," but in fact he has moved from reasoning to the assertion of faith. That is, the reasons he offers as evidence of death’s unimportance really do not in any way support the assertion that we live eternally, and it is this last assertion (if it is true) that most emphatically diminishes death, and this reveals his theme: our faith in an eternal existence makes death meaningless.

Death has no power.

what is the mood of the poem?

The persona very directly  addresses death and speaks to him like a real person, an evil person but, who really has no power.

The speaker dismisses death as a triviality.

 Death is the slave of "Fate, Chance, kings, and desperate men"; Death dwells with unsavory people ("Poison, War, and Sickness"); "poppy or charms can make us sleep as well." These negative things about death makes death look like nothing!

The speaker says that death has  no power at all and cannot " brag" or boast that he is in charge.

In Donne's poem, ' Valediction Forbidding Mourning' also death is looked upon as some thing like a sleep.