You can override modal style and the forms from your theme or child theme.  You can fine the modal styles in the wp-content/plugins/bp-ajax-registration/templates/bpajaxr directory.

An style is organized as

-Style Directory(.e.g default)
– – assets
– – – bpajaxr-style.css

– – panel-login.php
– – panel-registration.php
– – panel-forget-password.php

You can override any of the file by copying it to

Your_theme_or_child_theme/bpajaxr directory.

For example, if you want to override the default/assets/bpajaxr-style.css, you will copy it to yourtheme/bpajaxr/default/assets/bpajaxr-style.css.

Same applies to other files.