how do the phrases star-crossed lovers and death-mark'd love introduce the theme of love and fate in the prologue to romeo and juliet

It shows the irony of the love between Romeo and Juliet, who were never meant to be.

Explanation:

Star-crossed lovers is a phrase used to show that the lovers randomly met, and were never meant to fall in love without a complete coincidence. Death-mark'd portrays how their love is frowned upon by their opposing families and they're going to die for their love.