If you have specific poems you are studying, you will need to submit another question with the identification of those specific pieces.
To identify the major theme of a poem, look for the main idea that is being expressed. In poetry, this may not mean some thought or opinion that is directly stated - it may be expressed with symbolic imagery that needs to be thoughtfully interpreted. However, there will be a purpose for the writing of the poem, and that's what you are trying to identify as the theme.
The mood of the poet is often influenced by the poet's reaction to the theme of the poem. If you are reading a poem describing discrimination from the point of someone who is being left out or persecuted, the mood of the poet will reflect the hardships, if not outright physical danger, of being part of an endangered population. If you are reading about war, you may be dealing with a mood of sorrow over casualties or separations from friends and family, you may be reading about the intense experiences and emotions when involved in a battle, you may be reading about the trauma of dealing with the aftermath.