title: Material-UI theme.mixins.toolbar not working with Typescript
body: <p>I am using Material-UI for a React &amp; Typescript web site I am building.  It's been working great so far, but I have one very specific problem.  I am attempting to use one of the code examples, but it just does not seem to work with Typescript:</p>
  
  <p><a href="https://material-ui.com/components/app-bar/#fixed-placement" rel="nofollow noreferrer">https://material-ui.com/components/app-bar/#fixed-placement</a></p>
  
  <p>Specifically:</p>
  
  <pre><code>const useStyles = makeStyles(theme =&gt; ({
      offset: theme.mixins.toolbar,
  }));
  </code></pre>
  
  <p>Generates the following error:</p>
  
  <pre><code>(property) Mixins.toolbar: React.CSSProperties
  No overload matches this call.
    Overload 1 of 2, '(style: Styles&lt;Theme, {}, "offset"&gt;, options?: Pick&lt;WithStylesOptions&lt;Theme&gt;, "link" | "meta" | "flip" | "element" | "defaultTheme" | "name" | "media" | "index" | "generateId" | "classNamePrefix"&gt; | undefined): (props?: any) =&gt; Record&lt;...&gt;', gave the following error.
      Argument of type '(theme: Theme) =&gt; { offset: CSSProperties; }' is not assignable to parameter of type 'Styles&lt;Theme, {}, "offset"&gt;'.
        Type '(theme: Theme) =&gt; { offset: CSSProperties; }' is not assignable to type 'StyleRulesCallback&lt;Theme, {}, "offset"&gt;'.
          Call signature return types '{ offset: CSSProperties; }' and 'Record&lt;"offset", CSSProperties | CreateCSSProperties&lt;{}&gt; | ((props: {}) =&gt; CreateCSSProperties&lt;{}&gt;)&gt;' are incompatible.
            The types of 'offset' are incompatible between these types.
              Type 'CSSProperties' is not assignable to type 'CSSProperties | CreateCSSProperties&lt;{}&gt; | ((props: {}) =&gt; CreateCSSProperties&lt;{}&gt;)'.
                Type 'CSSProperties' is not assignable to type 'CreateCSSProperties&lt;{}&gt;'.
                  Index signature is missing in type 'CSSProperties'.
    Overload 2 of 2, '(styles: Styles&lt;Theme, {}, "offset"&gt;, options?: Pick&lt;WithStylesOptions&lt;Theme&gt;, "link" | "meta" | "flip" | "element" | "defaultTheme" | "name" | "media" | "index" | "generateId" | "classNamePrefix"&gt; | undefined): (props: {}) =&gt; Record&lt;...&gt;', gave the following error.
      Argument of type '(theme: Theme) =&gt; { offset: CSSProperties; }' is not assignable to parameter of type 'Styles&lt;Theme, {}, "offset"&gt;'.
        Type '(theme: Theme) =&gt; { offset: CSSProperties; }' is not assignable to type 'StyleRulesCallback&lt;Theme, {}, "offset"&gt;'.ts(2769)
  </code></pre>
  
  <p>I'm not sure how to resolve this.  In the Material-UI documentation they have a page about Typescript, but it does not provide clear direction about how to solve a problem like this:</p>
  
  <p><a href="https://material-ui.com/guides/typescript/#typescript" rel="nofollow noreferrer">https://material-ui.com/guides/typescript/#typescript</a></p>