How do you fix this pronoun agreement.

The first theme that the author demonstrates is to not let people with a disability feel abnormal, but for them to use the abilities that they have to help them build their self-confidence.

I don't see a problem with pronoun/antecedent agreement here.

Pronouns: them, they, them, their
All these pronouns are referring to "people" which is plural, as are the pronouns.

http://grammar.ccc.commnet.edu/grammar/pronouns.htm
See #2.

To fix the pronoun agreement in this sentence, we need to ensure that the pronouns used are in agreement with the nouns they refer to. In this case, we have the pronoun "them" referring to the noun "people with a disability."

The sentence currently reads:
"The first theme that the author demonstrates is to not let people with a disability feel abnormal, but for them to use the abilities that they have to help them build their self-confidence."

To maintain pronoun agreement, we can rephrase the sentence as:
"The first theme that the author demonstrates is to not let people with a disability feel abnormal, but to encourage them to use the abilities they have to help themselves build self-confidence."

In this revised sentence, the pronoun "them" agrees with the plural noun "people," and the possessive pronoun "themselves" agrees with the plural noun "abilities."