Via CSS file
WPS CRM supports custom.css file that will be autoloaded all over the CRM to apply your custom styles.
This file doesn’t exist in the core files and you won’t 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 and apply custom styles.
You will need some basic knowledge of CSS, you can read more here
Via Admin Area
If you have enabled the Theme Style module in Setup->Modules you can apply custom CSS in Setup->Theme Style->Custom CSS
Custom CSS can be added directly for the admin area, clients area, and both admin and clients area, depends on where you add the CSS, will be loaded accordingly to the section you added the code.