Create theme dir if needed (#49)

- Use Path.Combine throughout
- Update theme versions
- Update generator version

src/MyWebLog.Domain/ViewModels.fs

@@ -154,7 +154,9 @@ type DisplayTheme = {
     
     /// Create a display theme from a theme
     static member FromTheme inUseFunc (theme: Theme) =
-        let fileName = if string theme.Id = "default" then "default-theme.zip" else $"./themes/{theme.Id}-theme.zip"
+        let fileName =
+            if string theme.Id = "default" then "default-theme.zip"
+            else Path.Combine(".", "themes", $"{theme.Id}-theme.zip")
         { Id            = string theme.Id
           Name          = theme.Name
           Version       = theme.Version