I want to use a global Style in WP7, something like:
<Style TargetType="Button">
//some code here
</Style>
The problem is that this code does not seem to work in WP7.
I know how to add x:Key to the Style and after that how to reference it as a StaticResource, but this is not my case. I want a global Style.
If I understand you correctly you want to use Implicit Styles in WP7.
If so then have in mind that: Implicit Styles are a feature of Silverlight 4 (and WPF): Windows Phone 7 is based on Silverlight 3+(with a few Silverlight 4 features added).
Since there’s no Implicit Styles in Silverlight 3 this mean that there is no way to use them in Windows Phone 7 as well.
So, if you want to implement some kind of global Style in WP7, I would suggest that you try the approach with StaticResource as Matt Lacey suggested.