[material-ui][Filled Input] Undocumented non-themeable values

Steps to reproduce

Link to live example: https://github.com/mui/material-ui/blob/0b38955227cf4fc879fea5c8d23e167a5b9c4936/packages/mui-material/src/FilledInput/FilledInput.js#L45-L49
and e.g.
https://github.com/mui/material-ui/blob/0b38955227cf4fc879fea5c8d23e167a5b9c4936/packages/mui-material/src/FilledInput/FilledInput.js#L52

Nothing to reproduce.

Current behavior

After searching for hours where the background colors for the FilledInput come from, I finally started to read the code of the FilledInput and found that it has a wierd an completely undocumented way on how to theme it (or at least I was not able to find any documentation about it).

So maybe the issue is Make the FilledInput use variables from theme or
Missing documentation about

Expected behavior

Documentation on how to customize FilledInput or a FilledInput that uses default theme values

Further question:
Isn't the palette.action exactly for this case but not used?

Search keywords: FilledInput, background