migration notes

[skip ci]

docs/guides/migrations.md

@@ -14,6 +14,12 @@ The Intershop PWA now uses Node.js 18.16.0 LTS with the corresponding npm versio
 As a leftover adaption regarding the switch from deprecated class-based route guards in favor of functional guards the `addGlobalGuard` function was adapted to actually work with functional guards.
 If the `addGlobalGuard` function is used for customizations make sure it now works as expected.
 
+The two standard themes `b2b` and `b2c` where refactored so the `b2c` theme could be changed into a [configurable theme](./themes.md#configurable-theme) that uses CSS custom properties (CSS variables).
+Since SCSS color calculations do not work with CSS custom properties (they need real values instead of `var(--corporate-primary)`), SCSS functions like `darken()` and `lighten()` were removed from the standard Intershop PWA SCSS styling.
+Existing projects that do not want to use a configurable theme do not need to apply these changes to their custom styling.
+
+To use the new [configurable theme](./themes.md#configurable-theme) feature the feature toggle `extraConfiguration` needs to be enabled.
+
 ## 3.3 to 4.0
 
 The Intershop PWA now uses Node.js 18.15.0 LTS with the corresponding npm version 9.5.0 and the `"lockfileVersion": 3,`.
@@ -126,7 +132,7 @@ If you want to inject a token, use the methods `injectSingle` and `injectMultipl
 There is a new linting rule `useTypeSafeInjectionTokenRule` that enforces the usage of these methods.
 For more information, see the [Configuration Concept](../concepts/configuration.md#angular-cli-environments).
 
-We introduced the product notifications feature as a new extension which is toggled with the feature toggle 'productNotifications' in the `environment.model.ts`.
+We introduced the product notifications feature as a new extension which can be enabled with the feature toggle `productNotifications`.
 
 ## 3.2 to 3.3