Creating an unpublished Working Rough Draft

I am new to this and I am currently working on updating a dated WordPress theme / site and I had a question. Currently we have an event scheduled to take place in May, but I really would like to work on updating the website now, but not have it go down, or change drastically until the event is over.

My question, is there a way to play around with different themes and see what it would look like in a draft mode without actually publishing it for now?

Thanks a lot!