Add theme simple docs (#2452)

Author: alexcarpenter

docs/customization/themes.mdx

@@ -1,15 +1,16 @@
 ---
 title: Themes
-description: Clerk currently offers four prebuilt themes for you to customize the overall appearance of your Clerk app.
+description: Clerk currently offers six prebuilt themes for you to customize the overall appearance of your Clerk app.
 ---
 
-Clerk currently offers four prebuilt themes:
+Clerk currently offers six prebuilt themes:
 
 - [The default theme](#default-theme)
 - [The "shadcn" theme](#shadcn-theme)
 - [The "Dark" theme](#dark-theme)
 - [The "Shades of Purple" theme](#shades-of-purple-theme)
 - [The "Neobrutalism" theme](#neobrutalism-theme)
+- [The "Simple" theme](#simple-theme)
 
 ## Default theme
 
@@ -51,6 +52,24 @@ When using the [shadcn/ui](https://ui.shadcn.com/) library, you can use the `sha
   ![A sign-in form with a neobrutalist red theme](/docs/images/themes/neobrutalism.png){{ style: { maxWidth: '400px' } }}
 </div>
 
+## "Simple" theme
+
+This theme is a stripped down "Default" theme that removes some more advanced styling techniques, making it easier to apply your own custom styles.
+
+To use the simple theme, set `theme` to `simple`:
+
+```tsx {{ mark: ['simple'] }}
+<ClerkProvider
+  appearance={{
+    theme: 'simple',
+  }}
+/>
+```
+
+<div style={{padding: "1rem 0"}}>
+  ![A sign-in form with a simple theme](/docs/images/themes/simple.png){{ style: { maxWidth: '400px' } }}
+</div>
+
 ## Usage
 
 1. To get started, install the `@clerk/themes` package.