[AppBar] Add color transparent support (#19393)

* Fix `AppBar color='inherit'`

* Update docs

* [AppBar] Add color transparent support

Co-authored-by: Olivier Tassinari <olivier.tassinari@gmail.com>

docs/pages/api/app-bar.md

@@ -26,7 +26,7 @@ You can learn more about the difference by [reading this guide](/guides/minimizi
 |:-----|:-----|:--------|:------------|
 | <span class="prop-name">children</span> | <span class="prop-type">node</span> |  | The content of the component. |
 | <span class="prop-name">classes</span> | <span class="prop-type">object</span> |  | Override or extend the styles applied to the component. See [CSS API](#css) below for more details. |
-| <span class="prop-name">color</span> | <span class="prop-type">'default'<br>&#124;&nbsp;'inherit'<br>&#124;&nbsp;'primary'<br>&#124;&nbsp;'secondary'</span> | <span class="prop-default">'primary'</span> | The color of the component. It supports those theme colors that make sense for this component. |
+| <span class="prop-name">color</span> | <span class="prop-type">'default'<br>&#124;&nbsp;'inherit'<br>&#124;&nbsp;'primary'<br>&#124;&nbsp;'secondary'<br>&#124;&nbsp;'transparent'</span> | <span class="prop-default">'primary'</span> | The color of the component. It supports those theme colors that make sense for this component. |
 | <span class="prop-name">position</span> | <span class="prop-type">'absolute'<br>&#124;&nbsp;'fixed'<br>&#124;&nbsp;'relative'<br>&#124;&nbsp;'static'<br>&#124;&nbsp;'sticky'</span> | <span class="prop-default">'fixed'</span> | The positioning type. The behavior of the different options is described [in the MDN web docs](https://developer.mozilla.org/en-US/docs/Learn/CSS/CSS_layout/Positioning). Note: `sticky` is not universally supported and will fall back to `static` when unavailable. |
 
 The `ref` is forwarded to the root element.
@@ -49,6 +49,8 @@ Any other props supplied will be provided to the root element ([Paper](/api/pape
 | <span class="prop-name">colorDefault</span> | <span class="prop-name">.MuiAppBar-colorDefault</span> | Styles applied to the root element if `color="default"`.
 | <span class="prop-name">colorPrimary</span> | <span class="prop-name">.MuiAppBar-colorPrimary</span> | Styles applied to the root element if `color="primary"`.
 | <span class="prop-name">colorSecondary</span> | <span class="prop-name">.MuiAppBar-colorSecondary</span> | Styles applied to the root element if `color="secondary"`.
+| <span class="prop-name">colorInherit</span> | <span class="prop-name">.MuiAppBar-colorInherit</span> | Styles applied to the root element if `color="inherit"`.
+| <span class="prop-name">colorTransparent</span> | <span class="prop-name">.MuiAppBar-colorTransparent</span> | Styles applied to the root element if `color="transparent"`.
 
 You can override the style of the component thanks to one of these customization points:
 

packages/material-ui/src/AppBar/AppBar.d.ts

@@ -5,7 +5,7 @@ export interface AppBarProps extends StandardProps<PaperProps, AppBarClassKey> {
   /**
    * The color of the component. It supports those theme colors that make sense for this component.
    */
-  color?: PropTypes.Color;
+  color?: PropTypes.Color | 'transparent';
   /**
    * The positioning type. The behavior of the different options is described
    * [in the MDN web docs](https://developer.mozilla.org/en-US/docs/Learn/CSS/CSS_layout/Positioning).

packages/material-ui/src/AppBar/AppBar.js

@@ -69,6 +69,15 @@ export const styles = theme => {
       backgroundColor: theme.palette.secondary.main,
       color: theme.palette.secondary.contrastText,
     },
+    /* Styles applied to the root element if `color="inherit"`. */
+    colorInherit: {
+      color: 'inherit',
+    },
+    /* Styles applied to the root element if `color="transparent"`. */
+    colorTransparent: {
+      backgroundColor: 'transparent',
+      color: 'inherit',
+    },
   };
 };
 
@@ -83,8 +92,8 @@ const AppBar = React.forwardRef(function AppBar(props, ref) {
       className={clsx(
         classes.root,
         classes[`position${capitalize(position)}`],
+        classes[`color${capitalize(color)}`],
         {
-          [classes[`color${capitalize(color)}`]]: color !== 'inherit',
           'mui-fixed': position === 'fixed', // Useful for the Dialog
         },
         className,
@@ -116,7 +125,7 @@ AppBar.propTypes = {
   /**
    * The color of the component. It supports those theme colors that make sense for this component.
    */
-  color: PropTypes.oneOf(['default', 'inherit', 'primary', 'secondary']),
+  color: PropTypes.oneOf(['default', 'inherit', 'primary', 'secondary', 'transparent']),
   /**
    * The positioning type. The behavior of the different options is described
    * [in the MDN web docs](https://developer.mozilla.org/en-US/docs/Learn/CSS/CSS_layout/Positioning).