What is the main theme in Act 1, scene 3 of Romeo and Juliet?

The theme is marriage. More specifically, Juliet's marriage to the county Paris. At the beginning of the scene lady Capulet asks the nurse to summon Juliet to her chamber since she wishes to speak to her.

When Juliet arrives, her mother states:

Marry, that 'marry' is the very theme
I came to talk of. Tell me, daughter Juliet,
How stands your disposition to be married?

Juliet is fourteen and her mother wishes to know what she thinks of being wed. Juliet's answer is quite direct.

It is an honour that I dream not of.

Juliet's sarcasm is quite evident. She does not favour the idea at all. In an attempt to persuade Juliet to think differently, her mother uses two fallacies. Firstly, she uses an appeal to popular opinion:

... younger than you,
Here in Verona, ladies of esteem,
Are made already mothers:

In this, she asks that Juliet reconsider her point of view since many ladies of high rank and credibility, younger than Juliet, have already had children, obviously because they were married. Her second fallacy is an appeal to authority:

... by my count,
I was your mother much upon these years
That you are now a maid.

The point here is that as she was already a mother (and, therefore, married) for a time before she had even reached Juliet's age, she should therefore consider doing the same. Since she is her parent, Juliet should respect her authority and advice. Lady Capulet then informs Juliet that Paris had asked for her hand in marriage.

Lady Capulet and the nurse sing Paris's praises and paint a very attractive picture of the young man. He is good-looking and wealthy. Juliet will only benefit if she should become his bride.

Lady Capulet wishes to know from Juliet if she will be accommodating and consider Paris's proposal. Juliet replies as follows:

I'll look to like, if looking liking move:
But no more deep will I endart mine eye
Than your consent gives strength to make it fly.

Juliet is quite ambiguous in her response and says that she will look at Paris if her so looking will make her like what she sees or if the act of looking will please her mother. She, however, enedeavours not to look too deeply and will observe Paris for just as much as her mother permits her to.

In a parting remark, Lady Capulet reminds Juliet that the county Paris is waiting for her response and that she should keep him in her thoughts.

Juliet, the county stays.