style: Clean up dialogs stylesheet to fix layout issues

- remove unnecessary dialog type-specific padding, spacing and margin definitions that led to layout to problems
- bump the size of user avatar in authentication dialog
- nest css classes in the sass to better reflect the structure of dialog elements
- sync style of audio device selector with switcher-popup style
- sync text and padding styles with other elements
- fix incorrect button order in audio selection dialog

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7876
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3473>

js/ui/audioDeviceSelection.js

@@ -55,17 +55,18 @@ const AudioDeviceSelectionDialog = GObject.registerClass({
 
         this.contentLayout.add_child(content);
 
+        this.addButton({
+            action: () => this.close(),
+            label: _('Cancel'),
+            key: Clutter.KEY_Escape,
+        });
+
         if (Main.sessionMode.allowSettings) {
             this.addButton({
                 action: this._openSettings.bind(this),
                 label: _('Sound Settings'),
             });
         }
-        this.addButton({
-            action: () => this.close(),
-            label: _('Cancel'),
-            key: Clutter.KEY_Escape,
-        });
     }
 
     _getDeviceLabel(device) {