How to theoretically determine ESR of a capacitor and DCR of an inductor for a buck converter?
The ESR and DCR are mostly dependent on the package size of the component itself and thus determined entirely experimentally.  They are usually on the datasheet and they are one of the deciding factors that goes into choosing the component that goes on the circuit board.

Theoretical design of an SMPS is not the greatest idea since they are so fragile.  Theoretical design just doesn't take everything into account properly such as temperature, altitude, frequency, and so many other things.