refactor(themes): Improve createTheme to use themeParams and update types (#6778)

Author: alexcarpenter

.changeset/young-beds-sing.md

@@ -0,0 +1,5 @@
+---
+'@clerk/themes': patch
+---
+
+Improve return type of createTheme

packages/themes/src/createTheme.ts

@@ -1,10 +1,10 @@
 // Temp way to import the type. We will clean this up when we extract
 // theming into its own package
-import type { Appearance, BaseTheme, DeepPartial, Elements, Theme } from '@clerk/types';
+import type { BaseTheme, DeepPartial, Elements, Theme } from '@clerk/types';
 
 import type { InternalTheme } from '../../clerk-js/src/ui/foundations';
 
-interface CreateClerkThemeParams extends DeepPartial<Theme> {
+interface CreateClerkThemeParams extends DeepPartial<Theme>, Pick<BaseTheme, 'cssLayerName'> {
   /**
    * Optional name for the theme, used for telemetry and debugging.
    * @example 'shadcn', 'neobrutalism', 'custom-dark'
@@ -17,10 +17,10 @@ interface CreateClerkThemeParams extends DeepPartial<Theme> {
   elements?: Elements | ((params: { theme: InternalTheme }) => Elements);
 }
 
-export const experimental_createTheme = (appearance: Appearance<CreateClerkThemeParams>): BaseTheme => {
+export const experimental_createTheme = (themeParams: CreateClerkThemeParams) => {
   // Placeholder method that might hande more transformations in the future
   return {
-    ...appearance,
-    __type: 'prebuilt_appearance',
+    ...themeParams,
+    __type: 'prebuilt_appearance' as const,
   };
 };