[core] Accessibility - increase default contrastThreshold for WCAG AA…

… compliance (mui#34901)

Signed-off-by: Ken Bigler <kennethbigler@gmail.com>
Co-authored-by: Ken Bigler <ken_bigler@intuit.com>
Co-authored-by: Michał Dudak <michal.dudak@gmail.com>
Co-authored-by: siriwatknp <siriwatkunaporn@gmail.com>

docs/data/material/customization/color/color.md

@@ -109,3 +109,11 @@ const primary = red[500]; // #f44336
 const accent = purple['A200']; // #e040fb
 const accent = purple.A200; // #e040fb (alternative method)
 ```
+
+### Accessibility
+
+[WCAG 2.1 Rule 1.4.3](https://www.w3.org/WAI/WCAG21/Understanding/contrast-minimum.html) does recommend
+that you have a minimum of a 4.5:1 contrast ratio for the visual presentation of text and images of text.
+Material UI currently only enforces a 3:1 contrast ratio. If you would like to meet WCAG 2.1 AA compliance,
+you can increase your minimum contrast ratio as described in the
+[Theme customization](/material-ui/customization/palette/#accessibility) section.

docs/data/material/customization/palette/palette.md

@@ -130,9 +130,24 @@ type PaletteTonalOffset =
 
 A higher value for "tonalOffset" will make calculated values for "light" lighter, and "dark" darker.
 A higher value for "contrastThreshold" increases the point at which a background color is considered
-light, and given a dark "contrastText".
+light, and given a dark "contrastText". Note that "contrastThreshold" follows a non-linear curve, and
+starts with a value of 3 (requiring a minimum contrast ratio of 3:1).
 
-Note that "contrastThreshold" follows a non-linear curve.
+### Accessibility
+
+To meet the minimum contrast of at least 4.5:1 as defined in [WCAG 2.1 Rule 1.4.3](https://www.w3.org/WAI/WCAG21/Understanding/contrast-minimum.html), create a custom theme with `contrastThreshold: 4.5`.
+
+```js
+import { createTheme } from '@mui/material/styles';
+
+const theme = createTheme({
+  palette: {
+    // Used by `getContrastText()` to maximize the contrast between
+    // the background and the text.
+    contrastThreshold: 4.5,
+  },
+});
+```
 
 ### Example