You can choose the look and feel of your self-service portal by changing the CSS of your portal.
The Stylesheet tab under the Portal Customization page lets you add a custom CSS for your theme.
This acts as a single CSS file for the entire theme. It gets included in every page layout in your theme, and literally every page on your support portal. Simply group together CSS from multiple pages and paste it into the editor to see the changes take effect.
Try overriding the existing CSS of your existing Freshservice portal. You can do this by specifying CSS for an element or class.
A quick guide to using your custom CSS:
Go to Admin > Portals > Customize Portal.
Select the Stylesheet tab.
Enter your own CSS to start styling your portal.