Hi Benjamin,
We can reproduce the reported foreground color preservation issue with the given application. This issue was reproduced due to when the flag ApplyStylesOnApplication defined as true. This flag can be used to apply the Visual Style to all its child elements and its child windows. In this case, DataGrid has applied with the specified theme and we have used the textblock as child element for the HeaderCell to display text. So due to this flag get enabled, this textblock has been overridden with the default style, causes the foreground issue. We have logged the defect report for this issue and fix for this issue will be available in our 2018 Volume 2 release which will be available in end of May, 2018. At the moment, can you avoid to specify the ApplyStylesOnApplication flag as true for resolving the reported issue.
Regards,
Jai Ganesh