What is a Theme?
Theme means overall look, layouts, and style of the website or Application. In WPF when the style is used at Application Level it is known as the theme of the application.
WPF styles are used at various levels like control, layout, window, Application. When you define style at the application level it is accessible on the entire application.
Style is the same as the Style we are using in CSS. Style means assigning various properties to control and use that style whenever it is needed. Styles improves UI experience.
In WPF, the styles are defined in the resource dictionary. Each style has a unique key identifier and target type, inside the style tag we can define multiple setter tags to set different styles in a property. To use those styles all over the application, we will put styles inside the app.xaml file.