css/quick-settings: Update icon-button styles

- add an entire	flat style to button drawing function
- clean	up the icon button css in quick	settings
- adjust padding in QS sliders
- fixes #5918

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2510>

js/ui/quickSettings.js

@@ -198,7 +198,7 @@ var QuickSlider = GObject.registerClass({
 
         this._menuButton = new St.Button({
             child: new St.Icon({icon_name: 'go-next-symbolic'}),
-            style_class: 'icon-button',
+            style_class: 'icon-button flat',
             can_focus: true,
             x_expand: false,
             y_expand: true,