I want to improve the look of my program by using a custom created theme like this: http://www.mpgh.net/forum/33-visual-basic-programming/492570-biesis-vb-net-theme-archive.html
There is a way, though not recommended (depending on the complexity of your application). Make sure you've got a full backup of your project before trying this.
Ctrl + H.Find what enter the fully qualified name of the control to replace.Replace with enter the fully qualified name of the custom control.Look in to Current Project.Match case and Match whole word.Replace All button.