diff --git a/src/ThemeDialogUtils.cs b/src/ThemeDialogUtils.cs
index 074aa58..ae54022 100644
--- a/src/ThemeDialogUtils.cs
+++ b/src/ThemeDialogUtils.cs
@@ -1,4 +1,4 @@
-// This Source Code Form is subject to the terms of the Mozilla Public
+// This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
@@ -54,20 +54,40 @@ internal static string[] GetDisplayNames()
 
         internal static void LoadThemes(List<ThemeConfig> themes, ListView listView, ThemeLoadOpts opts)
         {
-            loadSemaphore.Wait(60000);
-            Size thumbnailSize = ThemeThumbLoader.GetThumbnailSize(listView);
-            ListViewItem focusedItem = null;
+            if (!loadSemaphore.Wait(60000))
+            {
+                LoggingHandler.LogMessage("Timed out waiting for theme load semaphore");
+                return;
+            }
 
-            foreach (ThemeConfig theme in themes.ToList())
+            try
             {
-                if (JsonConfig.settings.showInstalledOnly && !ThemeManager.IsThemeDownloaded(theme))
-                {
-                    continue;
-                }
+                Size thumbnailSize = ThemeThumbLoader.GetThumbnailSize(listView);
+                ListViewItem focusedItem = null;
 
-                try
+                foreach (ThemeConfig theme in themes.ToList())
                 {
-                    using (Image thumbnailImage = ThemeThumbLoader.GetThumbnailImage(theme, thumbnailSize, true))
+                    if (JsonConfig.settings.showInstalledOnly && !ThemeManager.IsThemeDownloaded(theme))
+                    {
+                        continue;
+                    }
+
+                    Image thumbnailImage;
+                    try
+                    {
+                        thumbnailImage = ThemeThumbLoader.GetThumbnailImage(theme, thumbnailSize, true);
+                    }
+                    catch (Exception exc)
+                    {
+                        // Unsupported or corrupt images can throw more than OutOfMemoryException.
+                        LoggingHandler.LogMessage("Failed to generate thumbnail for '{0}' theme: {1}", theme.themeId,
+                            exc);
+                        listView.Invoke(new Action(() =>
+                            ThemeLoader.HandleError(new FailedToCreateThumbnail(theme.themeId))));
+                        continue;
+                    }
+
+                    using (thumbnailImage)
                     {
                         listView.Invoke(new Action(() =>
                         {
@@ -93,27 +113,26 @@ internal static void LoadThemes(List<ThemeConfig> themes, ListView listView, The
                         }));
                     }
                 }
-                catch (OutOfMemoryException)
-                {
-                    ThemeLoader.HandleError(new FailedToCreateThumbnail(theme.themeId));
-                }
-            }
-
-            listView.Invoke(new Action(() =>
-            {
-                listView.Sort();
 
-                if (focusedItem == null)
+                listView.Invoke(new Action(() =>
                 {
-                    focusedItem = listView.Items[0];
-                }
+                    listView.Sort();
+
+                    if (focusedItem == null)
+                    {
+                        focusedItem = listView.Items[0];
+                    }
 
-                focusedItem.Selected = true;
-                listView.EnsureVisible(focusedItem.Index);
+                    focusedItem.Selected = true;
+                    listView.EnsureVisible(focusedItem.Index);
 
-                ThemeThumbLoader.CacheThumbnails(listView);
-            }));
-            loadSemaphore.Release();
+                    ThemeThumbLoader.CacheThumbnails(listView);
+                }));
+            }
+            finally
+            {
+                loadSemaphore.Release();
+            }
         }
 
         internal static void LoadImportedThemes(List<ThemeConfig> themes, ListView listView, ImportDialog importDialog)
@@ -135,13 +154,23 @@ internal static void LoadImportedThemes(List<ThemeConfig> themes, ListView listV
 
             Task.Run(() =>
             {
-                LoadThemes(themes, listView, new ThemeLoadOpts());
-
-                importDialog.Invoke(new Action(() =>
+                try
                 {
-                    importDialog.thumbnailsLoaded = true;
-                    importDialog.Close();
-                }));
+                    LoadThemes(themes, listView, new ThemeLoadOpts());
+                }
+                catch (Exception exc)
+                {
+                    LoggingHandler.LogMessage("Failed to load imported themes: {0}", exc);
+                }
+                finally
+                {
+                    // Close the import dialog even if thumbnail loading fails.
+                    importDialog.Invoke(new Action(() =>
+                    {
+                        importDialog.thumbnailsLoaded = true;
+                        importDialog.Close();
+                    }));
+                }
             });
         }