HTM was throwing an exception for ReadOnly files.

So, to fix this an explicit FileAccess had to be set. Package.Open uses
the default FileShare.None option. This causes a conflict and throws the
'UnauthorizedAccessException' exception. So, we just tell it that we are
there to read it and everything is fine.

src/htm/Main.cs

@@ -281,7 +281,7 @@ namespace thememan
 
             try
             {
-                using (Package zip = Package.Open(zipFile.FullName, FileMode.Open))
+                using (Package zip = Package.Open(zipFile.FullName, FileMode.Open, FileAccess.Read))
                 {
                     PackagePartCollection parts = zip.GetParts();