What is the central theme of the Bible?

The central theme of the Bible is JESUS CHRIST AND HIM CRUCIFIED!

If you read through the whole Bible from Genesis 1:1 to Revelation 22:21 with a seeking heart to know Christ and Him crucified, you will be blessed. For this is what the Lord declares in (Jeremial 29:13-14) : "You will seek Me and find Me when you seek Me with all your heart. I will be found by you. "

The Bible comprises 66 books, of which 39 books are in the Old Testament and 27 books in the New Testament. In short and concise words:
(1) The key message throughout the whole of Old Testament from Genesis to Malachi is PREPARATION OF CHRIST!
(2) The key message in the four Books of Gospel (Matthew, Mark, Luke and John) in the New Testament is MANIFESTATION OF CHRIST!
(3) The key message in the Book of Acts in the New Testament is PREACHING ON CHRIST!
(4) The key message in the 21 Epistles from Romans to Jude in the New Testament is EXPLANATION ON CHRIST!
(5) The key message in the Book of Revelation in the New Testament is The SECOND COMING OF CHRIST!

So focus on Christ as you read the Bible and you will get to know Him and what He did on the cross.