How to extend instead of overriding WPF Styles - wpf

Question

I want to use custom theme in my application and as far as I know I can accomplish this by using resource dictionary and referencing it in App.xaml. Styles would override the defaults like this:
<Style TargetType="{x:Type Label">
<Setter Property="Foreground" Value="Green" />
</Style>
Now as I guess the default Label style is overriden with same values but all my label fonts are green. The problem starts when I want to style one label somewhere again. When I want to change some other property in my Grid like this
<Grid.Resources>
<Style TargetType="{x:Type Label">
<Setter Property="FontSize" Value="28" />
</Style>
</Grid.Resources>
All labels inside my grid are losing their foreground color and have default one again (didn't I override defaults in previous step?). After some tries I found out that to do this properly i have to add another property to Style declaration BasedOn={StaticResource {x:Type Label}}" and it works. This is kind of weird for me because now I will have to repeat same BasedOn code in whole app and this is not how styling works - this should be done automatically! For example in HTML + CSS styles are inherited and merged and in WPF they are replaced...
Notice that when I don't use any styles controls still get their look from somehwere (System Themes?). How can I tell them to look for defaults somewhere else so without any additional code on styles they will think that they should be green by default?
Is there any way I can automate setting BasedOn property? Or maybe there is a better to do this overally?

Answer 1

I had the same problem. I used Zack's answer and improved it like following so if you don't specify a style the overridden default is still taken in account. It's basically what you would have done but just once in the ResourceDictionary.
<Window x:Class="TestWpf.RandomStuffWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Random Stuff Window">
<Window.Resources>
<ResourceDictionary>
<!-- Default Label style definition -->
<Style TargetType="{x:Type Label}">
<Setter Property="Foreground" Value="Green" />
</Style>
<!-- Extending default style -->
<Style TargetType="{x:Type Label}"
x:Key="LargeGreenForegroundLabel"
BasedOn="{StaticResource {x:Type Label}}">
<Setter Property="FontSize" Value="28" />
</Style>
</ResourceDictionary>
</Window.Resources>
<StackPanel>
<Button Click="Button_Click">Click</Button>
<Label Content="GreenForegroundLabel" /> <!-- Uses default style -->
<Label Style="{StaticResource LargeGreenForegroundLabel}"
Content="LargeGreenForegroundLabel" />
</StackPanel>
</Window>

Answer 2

Wpf has different levels of styles, that are applied in order of global > local. A style set directly on a control will override a style set globally, like in your example. I was trying to find a list of all the different places that a control looks for its styles but I cannot find one at the moment. As far as I know, you will have to use the BasedOn property to inherit a style and not completely override the properties of that style with the style you set locally.
Here is an example of a resource dictionary that has styles based on another style, so that you don't have do repeat the BasedOn binding over and over, you can just set the style on the specific element you want to have that style.
<Window x:Class="TestWpf.RandomStuffWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Random Stuff Window">
<Window.Resources>
<ResourceDictionary>
<Style TargetType="{x:Type Label}"
x:Key="GreenForegroundLabel">
<Setter Property="Foreground" Value="Green" />
</Style>
<Style TargetType="{x:Type Label}"
x:Key="LargeGreenForegroundLabel"
BasedOn="{StaticResource GreenForegroundLabel}">
<Setter Property="FontSize" Value="28" />
</Style>
</ResourceDictionary>
</Window.Resources>
<StackPanel>
<Button Click="Button_Click">Click</Button>
<Label Style="{StaticResource GreenForegroundLabel}"
Content="GreenForegroundLabel" />
<Label Style="{StaticResource LargeGreenForegroundLabel}"
Content="LargeGreenForegroundLabel" />
</StackPanel>
</Window>