[docs] Link to "Customization of Theme" from relevant theme interfaces (

#27689)

docs/src/pages/guides/typescript/typescript.md

@@ -24,6 +24,10 @@ The strict mode options are the same that are required for every types package
 published in the `@types/` namespace. Using a less strict `tsconfig.json` or omitting some of the libraries might cause errors. To get the best type experience with the types we recommend
 setting `"strict": true`.
 
+## Customization of `Theme`
+
+Moved to [/customization/theming/#custom-variables](/customization/theming/#custom-variables).
+
 ## Usage of `component` prop
 
 Many Material-UI components allow you to replace their root node via a `component`

packages/material-ui-private-theming/src/defaultTheme/index.d.ts

@@ -1,4 +1,5 @@
 /**
- * The default theme interface, augment this to avoid having to set the theme type everywhere
+ * The default theme interface, augment this to avoid having to set the theme type everywhere.
+ * Our [TypeScript guide on theme customization](https://material-ui.com/guides/typescript/#customization-of-theme) explains in detail how you would add custom properties.
  */
 export interface DefaultTheme {}

packages/material-ui/src/styles/createTheme.d.ts

@@ -18,6 +18,9 @@ export interface ThemeOptions extends SystemThemeOptions {
   unstable_strictMode?: boolean;
 }
 
+/**
+ * Our [TypeScript guide on theme customization](https://material-ui.com/guides/typescript/#customization-of-theme) explains in detail how you would add custom properties.
+ */
 export interface Theme extends SystemTheme {
   mixins: Mixins;
   components?: Components;