MUI TextField sx props does not apply styles

I'm trying to style a TextField component in a one-off fashion using the sx prop:

<TextField
    size="small"
    sx={{
        padding: '1px 3px',
        fontSize: '0.875rem',
        lineHeight: '1.25rem',
    }}
    {...params}
/>

I'm using MUI v5. If I inspect the input element, the styles are not applied. What am I missing?

UPDATE: it seems the styles are actually added to the wrapper element via its generated class. But I need to style the input element.

I've also tried using inputProps, but that did nothing at all.

MUI 5

You can style the constituent components by targeting their classes directly through sx. For example:

      <TextField
        label="My Label"
        defaultValue="My Value"
        size="small"
        sx={{
          ".MuiInputBase-input": {
            padding: '1px 3px',
            fontSize: '0.875rem',
            lineHeight: '1.25rem',
          }
        }}
      />

Or directly via inputProps.sx. For example:

      <TextField
        label="My Label"
        defaultValue="My Value"
        size="small"
        inputProps={{
          sx: {
            padding: "1px 3px",
            fontSize: "0.875rem",
            lineHeight: "1.25rem",
            color: "green",
          },
        }}
      />

Working CodeSandbox: https://codesandbox.io/s/customizedinputs-material-demo-forked-jog26e?file=/demo.js

Update for MUI 7

For MUI 7, inputProps is available via slotProps, so you can pass custom styling via sx a bit differently. For example:

      <TextField
        label="Outlined"
        defaultValue="My Value"
        size="small"
        slotProps={{
          inputLabel: {
            sx: {
              color: "red",
            },
          },
          input: {
            inputProps: {
              sx: {
                padding: "1px 3px",
                fontSize: "0.875rem",
                lineHeight: "1.25rem",
                color: "green",
              },
            },
          },
        }}
      />

Working CodeSandbox: https://codesandbox.io/p/sandbox/sad-alex-x85h7v?file=%2Fsrc%2FDemo.tsx%3A13%2C7-36%2C9