ANSWER: Your questions about who is God and what is the Bible's central theme seem simple enough. However, although they seem simple, they are actually difficult to answer since summaries are not always 100% accurate because qualifiers are omitted. That being said let us tackle your first question about our Creator.
God is a personal, all-powerful, all-knowing, eternal, loving, spirit-composed family (NOT a closed Trinity like so many think) who created everything that exists. What we know as the Godhead is actually composed of two spirit Beings. The first one is the Father and the second one is Jesus Christ (see John 10:30 - 31, 33, 17:20 - 23 and 1John 3:1 - 2).
The person believers refer to as Father is composed entirely of Spirit and gloriously reigns over all that exists as a Sovereign Divine Ruler. His perfect and loving will is accomplished through the power of His Holy Spirit (the Spirit is His power, not a separate Being). He possesses all authority and is omnipotent (all powerful). He, who has boundless grace and mercy, is also the very definition of love.
His holy and righteous character governs his existence. Although greater than Jesus Christ, he willingly shares everything his has with him. Jesus was sent by the Father to manifest His love for the world and to reveal His great and glorious plan for man.
The Being who became Jesus, before his birth, was (and is) a member of the Godhead and existed for all eternity with the Father. Jesus Christ created ALL things that exist. He is the God of the Old Testament who was the Lord and Lawgiver. He is revealed in the Gospels and the entire New Testament as the Word. In order to become the perfect sacrifice for sin and the Messiah who saves mankind, Christ willingly gave up His power and glorious existence as Deity to become a flesh and blood human being. He offered himself, through the cross, as the perfect atonement for man's sins.
The central theme
The Bible, all 1,189 chapters, is our Creator's special 'user manual' or instruction book made for human beings (his greatest creation). Its central theme is to delineate how humans can operate and live happily in this life with both the Beings that made them and their fellow human beings. This manual for man also shows humans how they can live forever AFTER death by revealing specifically how they should obey and have faith in the One who created them.
The Bible reveals how God is reaching down to humanity, through Jesus Christ, to reconcile them to Himself and redeem them from sin and spiritual ignorance so that they can have a perfect loving relationship with him and others FOREVER.