Thanks for the shared picture.
It seems that you are using a UI for WPF version from 2017, however the Fluent theme was introduced in the R1 2018 release. If you want to use that theme, you can upgrade to the R1 2018 version or to the latest one. Note, that we generally recommend using the latest version as it contains the most features/bug fixes.
I hope you find this information helpful.
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