hoverOpacity not exposed/defined in TypeAction interface for createPalette; causes typescript error when trying to override.

[rue-kchavan]

Issue: hoverOpacity field not defined in TypeAction interface used in createPalette.

Steps:

1. When trying to overrride the default theme by material ui as follows:

export default {
  palette: {
    primary: {
      main: '#354954',
    },
    action: {
      active: 'rgba(255, 255, 255, 0.20)',
      hover: 'rgba(255, 255, 255, 0.15)',
     hoverOpacity: 0.10,
    },
 }
}

Results in typescript error.

Object literal may only specify known properties, and 'hoverOpacity' does not exist in type 'Partial<TypeAction> | undefined'.```
Is there going to be a fix for this soon?

| Tech         | Version |
|--------------|---------|
| Material-UI  |  v0.20.1  |
| React        |    16.4.1     |
| browser      |  chrome |

[oliviertassinari]

@rue-kchavan You are right, we miss the hoverOpacity definition in:
https://github.com/mui-org/material-ui/blob/c937ab0f3b0a82b94de148d90d482534a542fc0c/packages/material-ui/src/styles/createPalette.d.ts#L11-L17

Do you want to submit a pull request to fix it?

[rue-kchavan]

Sure.