There is no particular Style property in the RadOutlookBar that you can use in this scenario for this custom Style. You can use entirely implicit approach - use NoXAML dlls, and use the OutlookBar MetroStyles implicitly in the Themes folder of your project (including your edited MinimezOutlookBarItemStyle). This is demonstrated in the first attached project.
Second attached project is a mixture of implicit styles and StyleManager. You can use normal dlls and set Application Metro Theme in code behind which sets the styles enough earlier and the implicit style in XAML ( your edited one) succeeds to apply after that. Note that if you don't use Application Theme and use xaml setting of theme like so:
this wont work - first your custom style applies then the Metro default styles override your custom one.
All the best,
the Telerik team