Hello Filip,

What you are seeing is to be expected since the title area is considered the "non-client" area of a window which is excluded. I believe updating our default styles, as explained on this forum thread will get the job done.

Note, our themes pertain only to framework elements. In addition, setting StyleMicrosoftControls to true will only apply if you are using an of the intrinsic Microsoft controls listed on our website, here.

www.infragistics.com/.../thememanager-applying-theme-to-supported-ms-wpf-controls

The non-client area of the window is based on the current OS theme.

Alternatively, reverting back using a Window and setting WindowStyle to None and creating your own menu bar by using a custom style as shown in the following msdn article might be the simplest option.

I recommend submitting a new product idea to simplify this matter in our ribbon window control. 

You can suggest new product ideas for future versions (or vote for existing ones) at our ideas page.
Submitting your idea will allow you to communicate directly with our product management team, track the progress of your idea at any time, see how many votes it got, read comments from other developers in the community, and see if someone from the product team has additional questions for you.
Remember when submitting your idea to explain the context in which a feature would be used and why it is needed as well as anything that would prevent you from accomplishing this today. You can even add screenshots to build a stronger case. Remember that for your suggestion to be successful, you need other members of the community to vote for it. You can also link back to this thread for additional details.

Let me know if you have any questions.

Thanks and have a great day!