padOsd: Don't hardcode blue color

Missed this one, since it's not set from CSS.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3480>

js/ui/padOsd.js

@@ -17,7 +17,7 @@ import * as Layout from './layout.js';
 
 import {loadInterfaceXML} from '../misc/fileUtils.js';
 
-const ACTIVE_COLOR = '#729fcf';
+const ACTIVE_COLOR = 'st-lighten(-st-accent-color, 15%)';
 
 const LTR = 0;
 const RTL = 1;