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I am not sure what was going wrong with your custom theme. In general the easiest way to load custom theme is to use the LoadPackageFile method:
This way you can very easy test you theme in a sample project, because you can test any changes momentarily. Then when your theme is ready you can add it as embedded resource to your project. More information is available in the following article: Using custom themes
Also I am not sure how you are copying the existing styles, but when you are editing the predefined themes
the best approach is to examine which elements have applied repository items (for which states also) and then change them where necessary. This way you will preserve the already created structure of applied styles and also the behavior of the controls will remain intact.
I hope this helps. Should you have any other questions do not hesitate to ask.