Steps to reproduce

Steps:

1. Create a custom theme using createTheme and override the MuiPickersDay component with a dayOutsideMonth style (for example, change the background color or text color).

const theme = createTheme({
  components: {
    MuiPickersDay: {
      styleOverrides: {
        dayOutsideMonth: {
          border: '1px solid red',
        },
      },
    },
  },
});

2. Wrap a date picker component (e.g., DatePicker from @mui/x-date-pickers) with a ThemeProvider that uses this custom theme.
3. Render the date picker and open its calendar view.

Current behavior

The styles defined in the dayOutsideMonth override were applied to .MuiPickersDay-root elements, but not to the .MuiPickersDay-dayOutsideMonth elements.

Expected behavior

The dayOutsideMonth style applied to days outside the current month.

Context

I'm trying to customize the MUI Date and Time pickers so that days outside the current month have a distinct style (for example, a faded color). I used the createTheme API to add a styleOverrides entry for MuiPickersDay.dayOutsideMonth. Despite specifying a style only for dayOutsideMonth, the custom style is being applied to every .MuiPickersDay-root element. I would like the dayOutsideMonth style to apply exclusively to days outside the current month.

Your environment

  System:
    OS: macOS 26.1
  Binaries:
    Node: 22.21.1 - /Users/kwokcheung.poon/.asdf/installs/nodejs/22.21.1/bin/node
    npm: 11.6.4 - /Users/kwokcheung.poon/dev/nokkel/nokkel-core/node_modules/.bin/npm
    pnpm: Not Found
  Browsers:
    Chrome: 143.0.7499.41
    Edge: Not Found
    Firefox: 145.0.2
    Safari: 26.1
  npmPackages:
    @emotion/react: ^11.14.0 => 11.14.0
    @emotion/styled: ^11.14.1 => 11.14.1
    @mui/core-downloads-tracker:  7.3.6
    @mui/material: ^7.3.6 => 7.3.6
    @mui/private-theming:  7.3.6
    @mui/styled-engine:  7.3.6
    @mui/system:  7.3.6
    @mui/types:  7.4.9
    @mui/utils:  7.3.6
    @mui/x-date-pickers: ^8.22.0 => 8.22.0
    @mui/x-internals:  8.22.0
    @types/react: ^18.2.67 => 18.3.27
    react:  18.3.1
    react-dom:  18.3.1
    typescript: 5.5.4 => 5.5.4

Search keywords: MuiPickersDay dayOutsideMonth style