fix(ts): make colorScheme optional (#4868)

Documentation says if it is not passed it will be "auto", but it is mandatory on this type.

Author: heyBett

packages/next-auth/src/core/types.ts

@@ -214,7 +214,7 @@ export interface NextAuthOptions {
  * [Pages](https://next-auth.js.org/configuration/pages)
  */
 export interface Theme {
-  colorScheme: "auto" | "dark" | "light"
+  colorScheme?: "auto" | "dark" | "light"
   logo?: string
   brandColor?: string
   buttonText?: string