doc: add note about css variables to migration guide

doc/MIGRATION_v3.md

@@ -1,6 +1,7 @@
 # V2 to V3 migration guide
 
 ## WebLN global object
+
 window.webln is no longer set by default. If you rely on WebLN being available in the global window object please add the following lines to a place in your application which will be called on every page:
 
 ```ts
@@ -33,6 +34,8 @@ This has been moved to `launchPaymentModal`.
 
 `<bc-modal>` (or `<Modal/>`) no longer needs to be rendered manually. Make sure to remove it so that the modal does not unexpectedly render on the page.
 
+> Make sure to set Bitcoin Connect css variables at the root (e.g. html/body selector) to ensure the modal uses the correct colors.
+
 ## Button
 
 Attributes have been moved from the button to the `init` function exposed by the API.