[libadwaita] stylesheet: Support headerbar.flat

[Alexander Mikhaylenko <alexm src gnome org>]

commit e7b03bd8d1075af0693054065c7588468576541e
Author: SeaDve <davecruz48 gmail com>
Date:   Tue Jul 27 11:14:23 2021 +0000

    stylesheet: Support headerbar.flat

 src/stylesheet/widgets/_header-bar.scss | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
---
diff --git a/src/stylesheet/widgets/_header-bar.scss b/src/stylesheet/widgets/_header-bar.scss
index fa813ada..b11c6dd2 100644
--- a/src/stylesheet/widgets/_header-bar.scss
+++ b/src/stylesheet/widgets/_header-bar.scss
@@ -56,6 +56,21 @@ headerbar {
     }
   }
 
+  &.flat {
+    background: none;
+    box-shadow: none;
+
+    min-height: 46px;
+
+    > windowhandle > box {
+      padding-bottom: 0;
+    }
+
+    &.default-decoration {
+      min-height: 36px;
+    }
+  }
+
   .solid-csd & {
     &:backdrop, & {
       &:dir(rtl), &:dir(ltr) { // specificity bump