[libadwaita/wip/exalm/accent: 5/5] doc: Mention the new accent color in migration guide

[Alexander Mikhaylenko <alexm src gnome org>]

commit edb5152a931423c85a4165880ba2a1afc935987c
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Mon Jun 14 18:21:53 2021 +0500

    doc: Mention the new accent color in migration guide

 doc/migrating-libhandy-1-4-to-libadwaita.md | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/doc/migrating-libhandy-1-4-to-libadwaita.md b/doc/migrating-libhandy-1-4-to-libadwaita.md
index 0d3e58de..c2674cca 100644
--- a/doc/migrating-libhandy-1-4-to-libadwaita.md
+++ b/doc/migrating-libhandy-1-4-to-libadwaita.md
@@ -251,3 +251,9 @@ colors have been removed:
 
 The `.list-button` style class has been renamed to the more accurate name
 `.outline`.
+
+The public colors `@theme_selected_bg_color` and `@theme_selected_fg_color` have
+been renamed to `@accent_color` and `@accent_text`.
+
+If you were using `@theme_selected_bg_color` as text color, use the `.accent`
+style class instead to make sure the text is readable.