[material-ui] Support CssVarsTheme in responsiveFontSizes return …

…type (@jxdp) (#42806)

Signed-off-by: jxdp <32483798+jxdp@users.noreply.github.com>
Co-authored-by: jxdp <32483798+jxdp@users.noreply.github.com>
Co-authored-by: ZeeshanTamboli <zeeshan.tamboli@gmail.com>

packages/mui-material/src/styles/responsiveFontSize.spec.ts

@@ -0,0 +1,16 @@
+import {
+  responsiveFontSizes,
+  experimental_extendTheme,
+  createTheme,
+  Theme,
+  CssVarsTheme,
+} from '@mui/material/styles';
+import { expectType } from '@mui/types';
+
+const cssVarsTheme = responsiveFontSizes(experimental_extendTheme());
+expectType<Omit<Theme, 'palette' | 'applyStyles'> & CssVarsTheme, typeof cssVarsTheme>(
+  cssVarsTheme,
+);
+
+const theme = responsiveFontSizes(createTheme());
+expectType<Theme, typeof theme>(theme);

packages/mui-material/src/styles/responsiveFontSizes.d.ts

@@ -1,6 +1,5 @@
 import { Breakpoint } from '@mui/system';
 import { Typography } from './createTypography';
-import { Theme } from './createTheme';
 
 export interface ResponsiveFontSizesOptions {
   breakpoints?: Breakpoint[];
@@ -9,7 +8,7 @@ export interface ResponsiveFontSizesOptions {
   variants?: Array<keyof Typography>;
 }
 
-export default function responsiveFontSizes(
-  theme: Theme,
+export default function responsiveFontSizes<T extends { typography: Typography }>(
+  theme: T,
   options?: ResponsiveFontSizesOptions,
-): Theme;
+): T;