Theme the message style preview area like a listbox so that it looks similar to the emoticon themes preview area.

im/content/instantbird/preferences/messagestyle.js

@@ -162,7 +162,7 @@ var previewObserver = {
     let themeName = document.getElementById("themename");
     if (themeName.value && !themeName.selectedItem)
       themeName.value = themeName.value;
-    previewObserver.browser = document.getElementById("browser");
+    previewObserver.browser = document.getElementById("previewbrowser");
     document.getElementById("showHeaderCheckbox")
             .addEventListener("CheckboxStateChange",
                               previewObserver.showHeaderChanged, false);

im/content/instantbird/preferences/themes.xul

@@ -96,7 +96,7 @@
                 </vbox>
               </hbox>
             </vbox>
-            <browser flex="1" id="browser" type="content-conversation"/>
+            <browser flex="1" id="previewbrowser" type="content-conversation"/>
           </deck>
           <hbox align="baseline">
             <label value="&messageStyleVariant.label;" control="themevariant"/>

im/themes/winstripe/instantbird/preferences-gnomestripe/preferences.css

@@ -170,6 +170,12 @@ radio[pane=paneAdvanced] {
   margin-top: 0;
 }
 
+/* Display the message theme preview browser with borders like a listbox */
+#previewbrowser {
+  -moz-appearance: listbox;
+  margin: 2px 4px;
+}
+
 /**
  * No message theme preview message box
  */

im/themes/winstripe/instantbird/preferences-pinstripe/preferences.css

@@ -255,6 +255,12 @@ caption {
   margin-top: 0;
 }
 
+/* Display the message theme preview browser with borders like a listbox */
+#previewbrowser {
+  -moz-appearance: listbox;
+  margin: 2px 4px;
+}
+
 /**
  * No message theme preview message box
  */

im/themes/winstripe/instantbird/preferences-winstripe/preferences.css

@@ -186,6 +186,12 @@ radio[pane=paneAdvanced][selected="true"] {
   padding-bottom: 4px;
 }
 
+/* Display the message theme preview browser with borders like a listbox */
+#previewbrowser {
+  -moz-appearance: listbox;
+  margin: 2px 4px;
+}
+
 /* No message theme preview message box */
 
 #noPreviewScreen {