reminders-section: Modernize popover

This makes the alarms popover look much closer to the AdwComboRow widget.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/399>

src/gui/event-editor/gcal-reminders-section.blp

@@ -24,13 +24,14 @@ Popover alarms_popover {
 
   styles [
     "menu",
+    "alarms",
   ]
 
   Box {
-    margin-top: 12;
-    margin-bottom: 12;
-    margin-start: 12;
-    margin-end: 12;
+    margin-top: 6;
+    margin-bottom: 6;
+    margin-start: 6;
+    margin-end: 6;
     orientation: vertical;
 
     $GtkModelButton event_start_button {

src/theme/style.css

@@ -46,6 +46,10 @@ calendar-view:selected {
   font-weight: bold;
 }
 
+popover.alarms modelbutton {
+  padding-top: 2px;
+  padding-bottom: 2px;
+}
 
 /* Event Popover */
 popover.event-popover,