Theme palette action selected color is not working in MUI version 5

When creating a custom theme, the palette.action.select color property is not working. The other action color properties are working great.

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

The select color on a dropdown select component is not working.

The default theme color is used instead.

Expected Behavior 🤔

The color on a selected dropdown menu item should be set.

Steps to Reproduce 🕹

See this code sample: https://codesandbox.io/s/palette-select-demo2-6ql8p?file=/src/Demo.tsx

import * as React from "react";
import { Box,InputLabel, MenuItem, FormControl, Select } from "@material-ui/core";
import { createTheme, ThemeProvider } from "@mui/material/styles";
import { red } from "@mui/material/colors";

const theme = createTheme({
  palette: {
    action: {
      select: red[500],
    },
  },
});

export default function Demo() {
  const [age, setAge] = React.useState('');

  const handleChange = (event) => {
    setAge(event.target.value);
  };

  return (
    <ThemeProvider theme={theme}>
      <Box sx={{ minWidth: 120 }}>
      <FormControl fullWidth>
        <InputLabel id="demo-simple-select-label">Age</InputLabel>
        <Select
          labelId="demo-simple-select-label"
          id="demo-simple-select"
          value={age}
          label="Age"
          onChange={handleChange}
        >
          <MenuItem value={10}>Ten</MenuItem>
          <MenuItem value={20}>Twenty</MenuItem>
          <MenuItem value={30}>Thirty</MenuItem>
        </Select>
      </FormControl>
    </Box>
    </ThemeProvider>
  );
}

Your Environment 🌎

Edge and Chrome browsers on Windows 10.

[bryanlopez-ibotta]

I am trying to adjust theme.palette.action.selectedOpacity and theme.palette.action.hoverOpacity but it seems hit or miss… For example, it affects Buttons with variant secondary but not primary? It also seems to have different effects if I set the primary color in main versus light or dark. What is theme.vars

'&:hover': _extends({
  textDecoration: 'none',
  backgroundColor: theme.vars ? `rgba(${theme.vars.palette.text.primaryChannel} / ${theme.vars.palette.action.hoverOpacity})` : alpha(theme.palette.text.primary, theme.palette.action.hoverOpacity),
  // Reset on touch devices, it doesn't add specificity
  '@media (hover: none)': {
    backgroundColor: 'transparent'
  }
},

[ctbeiser]

This is clearly a regression. MUI should be usable for design systems other than material, including ones that use secondary colors on list items.

At a minimum, it’s obviously wrong that there’s an item that can be specfied (selected) that changes nothing at all about the appearance of the theme.