[libadwaita/wip/exalm/dark: 5/6] doc: Mention HdyStyleManager in migration guide

[Alexander Mikhaylenko <alexm src gnome org>]

commit 57692457ca6577b9504af23b1125a51476dcd8a6
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Wed Sep 22 14:41:54 2021 +0500

    doc: Mention HdyStyleManager in migration guide

 doc/migrating-libhandy-1-4-to-libadwaita.md | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/doc/migrating-libhandy-1-4-to-libadwaita.md b/doc/migrating-libhandy-1-4-to-libadwaita.md
index 09c4fcba..8c091abb 100644
--- a/doc/migrating-libhandy-1-4-to-libadwaita.md
+++ b/doc/migrating-libhandy-1-4-to-libadwaita.md
@@ -77,6 +77,15 @@ where it's storing strings in GTK3, as the preferred replacement only exists in
 4, it can also be used with any other [struct@GObject.Value]. That use has no
 replacement and you can instead create your own objects to store those values.
 
+### Use `HdyStyleManager` instead of [property@Gtk.Settings:gtk-application-prefer-dark-theme]
+
+If your application is setting [property@Gtk.Settings:gtk-application-prefer-dark-theme]
+to `TRUE` to request dark appearance, consider setting `HdyStyleManager:color-scheme`
+to `HDY_COLOR_SCHEME_DARK_FORCE_LIGHT` or `HDY_COLOR_SCHEME_FORCE_DARK` instead,
+as needed.
+
+In libadwaita this will be the only way to request dark appearance.
+
 ## Changes that Need to Be Done at the Time of the Switch
 
 This section outlines porting tasks that you need to tackle when you get to the