Hey there @joanlee05850 -- thanks for your post! 

The only “customization” that you can truly achieve with your booking link is the text in the URL itself.

You can do this by: 

Heading into your event type to edit it > clicking “booking page options” > under “Event link” renaming the URL whatever you please > clicking “save and close.” 

If you are asking about customizing the look of the booking page, this is not possible on regular booking pages, but you can make some customizations when embedding your booking page on a website!
Check out our embed options here. Read more about setting up an embed here.

I hope this helps!