title: How to add custom colors name on Material UI with TypeScript?
body: <p>I am working with Material UI with TypeScript and want to add custom colors to my theme. Everything is working fine, except the VSCode linter that show me the next message.</p>
  <pre><code>Type '{ tan: string; lightRed: string; red: string; offBlack: string; white: string; }' is not assignable to type 'Partial&lt;CommonColors&gt;'.
    Object literal may only specify known properties, and 'tan' does not exist in type 'Partial&lt;CommonColors&gt;'.
  </code></pre>
  <p>In terms of develop and build is working fine, the only complain is the error message. I add a custom type to try to solve but it is not working.</p>
  <pre class="lang-js prettyprint-override"><code>const theme = createTheme({
    palette: {
      common: {
        tan,
        lightRed,
        red,
        offBlack,
        white,
      },
    },
  });
  </code></pre>
  <pre class="lang-js prettyprint-override"><code>import {
    PaletteOptions,
    CommonColors,
  } from '@material-ui/core/styles/createPalette';
  
  interface CustomColors extends CommonColors {
    tan: string?;
    lightRed: string;
    red: string;
    offBlack: string;
  }
  
  declare module '@material-ui/core/styles/createPalette' {
    export interface PaletteOptions {
      common: CustomColors;
    }
  }
  </code></pre>
  <p>I added to the <em>tsconfig.json</em> file. The tan, red and the rest of values are declared as Strings. Any clue about how can I solve this? Thanks in advance.</p>