We have a sample repo which uses the okta sign-in widget to customize your login experience.
It doesn’t use our spring-boot client library though.
You can take a look at the sample code here - https://github.com/okta/samples-java-spring-mvc
We also have an example application that uses the spring-boot library with custom login page - https://github.com/okta/okta-spring-boot/tree/master/examples/hosted-login-code-flow
@bdemers can help you with any questions you have. 