To incorporate themes look for small blocks of time you can do more caregiver run activities such as during freeplay gather the children for a short session and then send them off to play and then at the end have them clean up a little earlier than now and that will give you another short time to do an activity. In storytime read a book on the theme you want to do. Start by gradually setting aside periods in your day and the children will come to expect the new schedule quickly. As they get older or they are particulary interested in a topic they will want to start the sessions sooner or stay in the group activity longer and then it is time to adjust the schedule again to reflect what the children want.