For controls, the current style is often provided by a default style from control theming, or from styles generally applied to controls of that type by resources at page or application level (an implicit style). This property does not set or return default (theme) styles, but it does return either an implicit style or an explicit style that is acting on the element. In the case of implicit or explicit styles, it does not matter whether the style is specified as a resource or defined locally.
Setting the styles has some restrictions. You can reset the entire Style property to a new Style at any time, which will force a layout recomposition. However, as soon as that style is placed in use by a loaded element, the Style should be considered sealed. Attempting to make a change to any individual property of an in-use style (such as anything within the collection of Setters) causes an exception to be thrown. A style that is defined in markup is considered to be in use as soon as it is loaded from a resource dictionary (for resources), or the page it is contained within is loaded (for inline styles).
Style is a dependency property with special precedence. The locally set style generally operates at the highest precedence in the property system. If the Style is null at this point, during loading the property system checks for implicit styles in local or application resources that specify that type. If the style is still null after this step, then the acting style for presentation purposes generally comes from the default (theme) style, but the default style is not returned in the Style property value. See Dependency Property Value Precedence or Styling and Templating.