On the K2 user profile, there are fields that are un-needed for my website like regions, country, website, favorite book, date of birth. How can I remove these fields?
Also is there a way to change the theme of the user profile page? it looks boring

Go to Extension -> Plugin Manager and locate the User Profile plugin. In your case this should be Enabled. There you can choose which of the fields you want to show or not.

You can make any change to the layout with CSS.
The profile page is located in the com_k2 template folder. You can create an override of the template and make any modification if CSS is not enough.

Read here getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates how to create/use the overrides.