content-message-row.ui: Remove displayname style, use adw styling

Remove the custom display name style and instead use
libadwaita's "heading" and "accent" styles to set the right styling and colors.

data/resources/style.css

@@ -122,15 +122,9 @@ headerbar.flat {
   font-weight: bold;
 }
 
-.displayname {
-  font-size: 0.9em;
-  font-weight: bold;
-  color: @theme_selected_bg_color;
-}
-
 .timestamp {
-  font-size: 0.9em;
   min-width: 36px;
+  font-weight: normal;
 }
 
 .codeview {

data/resources/ui/content-message-row.ui

@@ -24,13 +24,15 @@
                     <property name="ellipsize">end</property>
                     <property name="selectable">True</property>
                     <style>
-                      <class name="displayname"/>
+                      <class name="heading"/>
+                      <class name="accent"/>
                     </style>
                   </object>
                 </child>
                 <child type="end">
                   <object class="GtkLabel" id="timestamp">
                     <style>
+                      <class name="heading"/>
                       <class name="timestamp"/>
                       <class name="dim-label"/>
                     </style>