[codemod] Add theme-v6 migration (#42056)

packages/mui-codemod/README.md

@@ -1099,8 +1099,74 @@ npx @mui/codemod@next deprecations/step-connector-classes <path>
 
 ### v6.0.0
 
+#### `theme-v6`
+
+```bash
+npx @mui/codemod@next v6.0.0/theme-v6 <path>
+```
+
+Update the theme creation from `@mui/system@v5` to be compatible with `@pigment-css/react`.
+
+- replace palette mode conditional with `theme.applyStyles()`
+- replace `ownerState` with `variants`
+- move theme variants to the root slot
+
+```diff
+  createTheme({
+    components: {
+      MuiButton: {
+-        variants: [
+-          {
+-            props: { color: 'primary' },
+-            style: {
+-              color: 'red',
+-            },
+-          },
+-        ],
+        styleOverrides: {
+-          root: ({ theme, ownerState }) => ({
++          root: ({ theme }) => ({
+            ...ownerState.variant === 'contained' && {
+              backgroundColor: alpha(theme.palette.primary.main, 0.8),
+              ...theme.palette.mode === 'dark' && {
+                backgroundColor: alpha(theme.palette.primary.light, 0.9),
+              }
+            },
++           variants: [
++             {
++               prop: { variant: 'contained' },
++               style: {
++                 backgroundColor: alpha(theme.palette.primary.main, 0.8),
++               },
++             },
++             {
++               prop: { variant: 'contained' },
++               style: {
++                 ...theme.applyStyles('dark', {
++                   backgroundColor: alpha(theme.palette.primary.light, 0.9),
++                 })
++               },
++             },
++             {
++               prop: { color: 'primary' },
++               style: {
++                 color: 'red',
++               },
++             },
++           ],
+          })
+        }
+      }
+    }
+  })
+```
+
 #### `styled-v6`
 
+```bash
+npx @mui/codemod@next v6.0.0/styled-v6 <path>
+```
+
 Updates the usage of `styled` from `@mui/system@v5` to be compatible with `@pigment-css/react`.
 
 This codemod transforms the styles based on props to `variants` by looking for `styled` calls:

packages/mui-codemod/src/util/getReturnExpression.js

@@ -0,0 +1,14 @@
+/**
+ * @param {import('ast-types').namedTypes.ArrowFunctionExpression | import('ast-types').namedTypes.FunctionExpression} node
+ */
+export default function getReturnExpression(node) {
+  let body = node.body;
+  if (body === 'BlockStatement') {
+    body = body.body;
+  }
+
+  if (Array.isArray(body)) {
+    return body.find((statement) => statement.type === 'ReturnStatement')?.argument;
+  }
+  return body;
+}