Is there a way to customize CiviCRM UI?
This is a really broad question, and without some guidance as to what you consider a "professional CRM applications" looks like it is tricky one to answer.

That said, with Drupal we often built out a UI via Views and Webforms that can keep all the but the hardcore admins out of civicrm entirely  - depending of course on what the extent of their requirements are.

This may be an easier path for you to explore than rebuilding the civicrm ui directly.