Perfex CRM supports custom.css file that will be autoloaded all over the CRM to apply your custom styles.
This file don’t exists in the core files and you wont need to worry to lose your changes when performing an update.
You can achieve this by creating a file custom.css in assets/css, after the file is created you can use your favorite code editor an apply custom styles.
You will need some basic knowledge of CSS, you can read more here