The theme of the poem is about courage in the fight against fear, worried about the worst that's gonna happen in the dark.
In the first stanza;
“Why are we so afraid of the dark?
It doesn’t bite and doesn’t bark
Or chase old ladies round the park
Or steal your sweeties for a lark”
Means Asking why people are so afraid of the dark when darkness does nothing, but human minds make the thought that there must be evil in every darkness.
The color in this poem it has an atmosphere of darkness and fearful in the line " Why are we so afraid of the dark? ", the audience imaginary that the dark place always make people afraid.
"The dark is cosy, still and calm
And never does you any harm"
In these lines, darkness also gives calmness, warmth and That darkness is not evil as much as we imagine.