This guide gives the steps you need to take if you’re looking to customise the styles of the Client Portal.
Firstly, you need to find the WordPress page ID of your client portal page
WordPress adds a body class to all pages with their ID as follows page-id-86579 or parent-pageid-86579 if using custom child pages as tabs
Once you’ve found your class selectors, you can add custom CSS to your theme (usually via the Theme Customiser).
Alternatively you can add additional CSS using a third party plugin like this one or you can add custom CSS to your theme through Jetpack.