Q. What are the main themes of the respective Vedas?
The Vedas are theistic in nature and emphasise to know and adore God and act according to his eternal laws. They possess all true knowledge, and all that which can be know through knowledge. The four Vedas (Rg, Yaju, Sam, Atharv) have predominantly have four main themes, viz., knowledge, activity, worship and human conduct including scientific and management technology (respectively).
They provide all primary and causatic knowledge about God, human beings, nature, their activity, and relationships. All sorts of knowledge in every field of human activities, physical sciences and technologies are dealt there in brief. The Vedas contain all basic material, ethical, and logical principles inspiring practical teachings and ideals for daily needs which can help human beings in elevating them to te supreme state, i.e, Moksha, individually and discharging social responsibilities.
Broadly speaking, there are two types of knowledge in the Vedas - PARA and APARA. PARA deals with the concepts of God, His manifestations of powers and spiritualism while APARA is concerned with wordly matters and pleasures - thus covering the entire field of knowledge.
Maharshi Vyaas in Mahabharata says "Whatever texts of different subjects are there in the world, they owe their origin to the Vedas". Rishi YagyaValkya in his smriti says "All shastras have originated from the eternal Vedas only, and no book is comparable to the Vedas". Manu in his smriti reiterates "The primary source of all knowledge of the past, present and future are from the Vedas. This knowledge is eternal, truthful and authentic"
Does the Veda contain detailed scientific knowledge for immediate use?
What are the objectives and teachings of the Vedas?
what are the introductoy books on the Vedas?
Does Vedas contain Sciences?
Yes, Vedas contain all forms of fundamental knowledge – physics, chemistry, medicines, sociology, mathematics, theology, engineering etc etc. For details please refer the link - Science in Vedas
Q.~ How should we pray to the GOD as per Vedas?
Vedas do not conceive of a God or Paramatma who would change his course of actions even a bit to accommodate our prayers. Then what is the use of such prayers or innumerable similar ones, which He would never listen to?
Some important teachings of Vedic Granth are as below in the Sub Pages and Attachments...