Windows 10 XAML 'Theming' a Style - xaml

Question

I want to make ComboBoxes in my Windows 10 App to have theme colours (One have a green theme, one with an orange theme...etc). I can do it by making different styles for each different colour, but that adds a huge amount of markup and it's a mess to manage. Because they're also the styles, I read that bindings won't work because styles are sealed once they're initialized.
Has anyone figured out a way to just change colours of a style without having to make multiple separate styles?

Answer 1

WPF has some nice markup for this very problem. You can use DynamicResource to allow the style to refresh every time the resource gets changed. Here's an example:
<SolidColorBrush x:Key="ColourAccent">#448AFF</SolidColorBrush>
<Style TargetType="Button">
<Setter Property="Background" Value="{DynamicResource ColourAccent}"/>
</Style>
This of course is a very simple implementation of this method, but essentially the DynamicResource is a reference to ColourAccent that, if the resource was to change, will automatically reflect the change in the style.
Now, changing the resource is a slightly different problem, for theming your styles, you need somewhere to put all of your colours, the solution here is to use multiple ResourceDictionaries and using MergedDictionaries. Let me show you what I mean:
Firstly, add a folder to your project called Themes, it'll just make things easier. Also, add that Style I mentioned earlier to either the Window.Resources or App.Resources, it's pretty important.
You need to add a ResourceDictionary file to the folder (Call it Amber.xaml), it'll look something like this:
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<!-- Amber -->
<SolidColorBrush x:Key="ColourAccent">#FFC107</SolidColorBrush>
</ResourceDictionary>
Here is a simple resource dictionary, it contains one resource called ColourAccent, now we need to create another one (Call it Blue.xaml), but with a different colour:
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<!-- Blue -->
<SolidColorBrush x:Key="ColourAccent">#448AFF</SolidColorBrush>
</ResourceDictionary>
The magic here will become clear soon enough. Having multiple resource dictionaries with resources under the same name will allow you to create themes. The method of which is to swap the current resource dictionary for a new one, and by doing so, the DynamicResource will notice there has been a change and it'll refresh the control styles.
Now you have two resource dictionaries, you need to work out which dictionary to use. Firstly, we need to define the default resource dictionary that your application is going to use, you need to declare this in App.xaml.
<Application ...>
<Application.Resources>
<ResourceDictionary>
...
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/YourNamespace;component/Themes/Amber.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>
So here you'll see the MergedDictionaries element, this is where the magic happens. If you run the app now, you should see that any buttons will have a background colour of Amber. Cool, huh?
Now it gets a little bit trickier, we will be removing the resource dictionary that is currently in the dictionary and replacing it with a different theme (a different resource dictionary).
Here is some C# to do that:
Disclaimer: I just threw this code together, there is almost certainly a better way, but you get the idea. Put the following code in a mouse click event, or somewhere where you can step it through to see what's happening.
Firstly, remove the current theme from the merged dictionaries.
//Find the current dictionary
ResourceDictionary oldDictionary = App.Current.Resources.MergedDictionaries.FirstOrDefault();
//If we found one, remove it.
if (dictionary != null)
App.Current.Resources.MergedDictionaries.Remove(oldDictionary);
Now we just need to add a different resource dictionary. I'm just going to add the Blue theme for the sake of an example:
//Declare some variables.
string folderPath = "/YourNamespace;component/Themes/";
string desiredTheme = "Blue";
//Create the new resource dictionary
ResourceDictionary newDictionary = new ResourceDictionary();
newDictionary.Source = new Uri(string.Format("{0}{1}.xaml", folderPath, desiredTheme), UriKind.RelativeOrAbsolute);
//Add the resource dictionary to the merged dictionaries.
App.Current.Resources.MergedDictionaries.Add(newDictionary);
Now if all goes well, the background for any buttons in your app should now be Blue. Hooray!
Using this method, which in essence is creating multiple resource dictionaries each with resources under the same name, you can create multiple themes for your application. This isn't restricted to colours, you can have entire styles which are theme specific, where one theme might display a button in a completely different way as another style. Experiment and see what you can come up with. Good luck!