@Ben Gribaudo mentioned that a bug does not allow the <Application.Resources> to connect properly. In his blog: http://bengribaudo.com/blog/2010/08/19/106/bug-single-application-resources-entry-ignored he mentioned a fix where you add a 'dummy' style entry as the bug seems to take effect when only one entry is defined.

I added <Style x:Key="unused because Application.Resources does not load if only one entry is defined." /> at then bottom under my button style which did the trick. Shame that a bug reported in 2010 is still here almost a month to 2025.