What main ideas are conveyed in the poem?

There is a sense of darkness of both mood and imagrery.

The day is cold, and dark, and dreary;
It rains, and the wind is never weary;

This darkness extends to the speaker's life,

My life is cold, and dark, and dreary;
It rains, and the wind is never weary;

By the third stanza, the speaker has a sense of hope. The metaphor being that bad times eventually pass.

Be still, sad heart! and cease repining;
Behind the clouds is the sun still shining;