What are the climax and themes of Shakespeare's Romeo and Juliet?

The climax or point of highest interest in the plot of Shakespeare's Romeo and Juliet occurs in Act III, Scene 1 when Romeo kills Tybalt after the death of Mercutio. Romeo is subsequently banished, leading directly to the falling action and resolution of the play when Romeo and Juliet commit suicide. There are several themes in Romeo and Juliet, but the three that seem to stand out are the contrast between love and hate, the power of fate and the contrast of light and dark. 

The contrast between love and hate is at its most extreme in the middle of the play as Romeo and Juliet marry each other in Act II, Scene 6. The wedding is quick and secret, but there seems to be no denying the fact that the two young people are blissfully in love and grateful to be married. This act of ultimate love is immediately followed in Act III, Scene 1 by the violence of hate as Tybalt kills Mercutio and is, in turn, killed by Romeo in a fit of revenge.

The power of fate, which is often referred to as the "stars," pervades the plot. It is first pronounced in the Prologue as the audience is informed that "A pair of star-crossed lovers take their life." It is also hinted at in different sections as both Romeo and Juliet entertain the idea that their love may eventually lead to death. In fact, Romeo foreshadows his own death in Act I, Scene 4 as he claims that his attendance at the Capulet party may be the beginning of the end:

I fear too early, for my mind misgives
Some consequence yet hanging in the stars
Shall bitterly begin his fearful date
With this night’s revels, and expire the term
Of a despisèd life closed in my breast
By some vile forfeit of untimely death.

The imagery of light and dark is also pervasive. In Act I, Scene 1, Romeo is said to go into his room and shut out the light because he is in love with a girl who does not return his affection:

Away from light steals home my heavy son
And private in his chamber pens himself,
Shuts up his windows, locks fair daylight out,
And makes himself an artificial night.

When Romeo first sees Juliet, she lights up his world and "teaches the torches to burn bright." Throughout the balcony scene (Act II, Scene 2), Romeo again uses the the metaphor of light to portray Juliet. The imagery of light and dark, however, is sometimes juxtaposed in the play. In Act III, Scene 5, as Romeo and Juliet are waking up from their honeymoon night, the dark, in the form of the nightingale, is the couple's ally. It is the lark, "the herald of the morn" and daylight, which is their enemy.