Customize Keycloak login/signup pages

Is there a way to completely customize the pages in keycloak? For example if the user is redirected to the Sign In URL, I want to use my own pages that I’ve made in React.js

Yes.

For example: Serendipity Keycloak Theme

No.

You actually can use React for your login page. Right now I can’t recommend it. It would slow down the page in low bandwidth situations. React is just overkill for something as simple as a login. But since it has never been tried, I can’t say for sure that it won’t work out.

If you really want to do it there is nothing stopping you from changing login.ftl so that it loads a React app. We are doing this kind of thing for the new Account Console and you could do it for login as well.