There are 2 approaches to customize the theme -> directly override the resources in the BlueResources dictionary or create a custom resource dictionary and merge it instead of the BlueResources one.

If you take the first approach in a .NET standard project and override one of the colors -> an ArgumentException is thrown:

Use the second approach as a workaround where you create a custom resource dictionary, copy all the colors from the original one and modify them within your custom one.

Available in the R3 2018 SP release.