Answer:
I would say during her time in the Annex, the themes were a mixture of Isolation and Love.
Step-by-step explanation:
She felt isolated, she couldn't even run or jump, she was stuck inside all day long just so she could stay safe and the only people she ever talked to were the same ones that lived in the Annex with her along with a few others. The second, love, comes from her falling in love with Peter Van Daan. She knew she was falling in love with him and she was explaining in the diary how much she loved spending time with him and couldn't wait for the next day when they could do it all over again.
I really hope this helps you out!
Have an amazing day! :)