[gnome-control-center] Use @theme_fg_color for .drag-handle

[Robert Ancell <rancell src gnome org>]

commit 3493b50be9eaeb9f3a5e6dc7bced5c3e332810e8
Author: Ian Santopietro <isantop gmail com>
Date:   Tue Oct 15 18:54:30 2019 +0000

    Use @theme_fg_color for .drag-handle
    
    Instead of using @borders for drag handles, we use @theme_fg_color at 40% opacity. Because these are 
widgets intended to be interacted with by users instead of drawing borders, we should use a semantically 
appropriate color which updates according to whether a user has a dark theme selected or not. This ensures 
that the drag handles contrast with the background no matter what stylesheet the user is using.
    
    This improves the contrast of these widgets in the light theme from 1.7:1 to 2.9:1, which is still rather 
low, but greatly improves the usability of these widgets without having too large an impact on the look of 
the widgets in-situ. The contrast ratio is unchanged in the HighContrast style (4.0:1).
    
    This also improves the situation if we eventually adopt a dark style preference for users, and improves 
the portability of the application.

 shell/style.css | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/shell/style.css b/shell/style.css
index a548d6561..7ffc6ab06 100644
--- a/shell/style.css
+++ b/shell/style.css
@@ -1,3 +1,7 @@
 .drag-handle {
-    color: @borders;
+  color: alpha(@theme_fg_color, 0.4);
+}
+
+.drag-handle:backdrop {
+  color: alpha(@theme_unfocused_fg_color, 0.4);
 }