I have solved my problem by setting up display property to block (default is none):
form[id*=give-form] #give_terms_agreement input[type=checkbox], div[id*=give-form] #give_terms_agreement input[type=checkbox] {
display: block;
}
This error in console, helped me to solve the problem:
An invalid form control with name='give_agree_to_terms' is not focusable.
And indeed, it was not focusable because it was not visible.