Scenes in plays do not have themes so I will speak about the focus or purpose of the scenes.
The purpose of Act V, scene 2 is to show us that men like Lennox are deserting Macbeth and joining forces with Malcolm's army. If you recall, Birnam Wood is critical to the predictions. The army is meeting at Birnam Wood for their final assault on Dunsinane.
The next scene is important for two reasons. First we find a confident Macbeth ready to fight to the death and also we find out that Lady Macbeth is incurablely ill. The doctor explains that her illness is something only she can cure. It is beyond any physician's skill.
Remember, a theme is much greater. It is the main point being made about the entire play. For example, the theme of power corrupts but absolute power corrupts absolutely is a theme of this play.
These two scenes help to clarify what is going on in Scotland. Malcolm, the rightful king, has returned with a mighty army to claim his throne and men are flocking to his banner at Birnam Wood. We further learn that Lady Macbeth, who we have not seen for a long time, is mentally ill. Macbeth is ready, or so he thinks, for the coming battle.
At the beginning of the play men followed Macbeth because he was a great warrior. By the end of the play they are leaving him in droves because he is a bad leader. He rules by fear which is not good.