Added an example of how to extend the styles.

Author: reduckted

demo/VSSDK.TestExtension/ToolWindows/ThemeWindow/CustomizedStylesDialog.xaml

@@ -0,0 +1,61 @@
+<platform:DialogWindow 
+    x:Class="TestExtension.CustomizedStylesDialog"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
+    xmlns:platform="clr-namespace:Microsoft.VisualStudio.PlatformUI;assembly=Microsoft.VisualStudio.Shell.15.0"
+    xmlns:toolkit="clr-namespace:Community.VisualStudio.Toolkit;assembly=Community.VisualStudio.Toolkit"
+    xmlns:shell="clr-namespace:Microsoft.VisualStudio.Shell;assembly=Microsoft.VisualStudio.Shell.15.0"
+    mc:Ignorable="d" 
+    d:DesignHeight="450" 
+    d:DesignWidth="800"
+    toolkit:Themes.UseVsTheme="True"
+    Width="300"
+    Height="400"
+    Title="Customized Styles"
+    WindowStartupLocation="CenterOwner"
+    ShowInTaskbar="False"
+    >
+
+    <Window.Resources>
+        <ResourceDictionary>
+            <!-- 
+            To customize a style provided by the toolkit, you need to make the resources 
+            available by including the resource dictionary from the toolkit:
+            -->
+            <ResourceDictionary.MergedDictionaries>
+                <ResourceDictionary Source="{x:Static toolkit:ToolkitResourceKeys.ThemeResourcesUri}"/>
+            </ResourceDictionary.MergedDictionaries>
+
+            <!--
+            Now you can create your own styles that are based on the toolkit's styles.
+            -->
+            <Style TargetType="TextBox" BasedOn="{StaticResource {x:Static toolkit:ToolkitResourceKeys.TextBoxStyleKey}}">
+                <Setter Property="BorderBrush" Value="Red" />
+            </Style>
+
+            <!--
+            The toolkit only defines a few customized styles. Most styles come from the default Visual Studio
+            styles. These can be referenced using the keys from `Microsoft.VisualStudio.Shell.VsResourceKeys`.
+            -->
+            <Style TargetType="Button" BasedOn="{StaticResource {x:Static shell:VsResourceKeys.ThemedDialogButtonStyleKey}}">
+                <Setter Property="Foreground" Value="Green" />
+            </Style>
+        </ResourceDictionary>
+    </Window.Resources>
+
+    <StackPanel Orientation="Vertical" Margin="10">
+        <TextBox 
+            Text="This TextBox uses the style from the toolkit, but extends it to have a red border."
+            AcceptsReturn="True"
+            Height="50"
+            TextWrapping="Wrap"
+            />
+
+        <Button
+            Content="Green text on a button"
+            Margin="0,10,0,0"
+            />
+    </StackPanel>
+</platform:DialogWindow>

demo/VSSDK.TestExtension/ToolWindows/ThemeWindow/CustomizedStylesDialog.xaml.cs

@@ -0,0 +1,12 @@
+using Microsoft.VisualStudio.PlatformUI;
+
+namespace TestExtension
+{
+    public partial class CustomizedStylesDialog : DialogWindow
+    {
+        public CustomizedStylesDialog()
+        {
+            InitializeComponent();
+        }
+    }
+}

demo/VSSDK.TestExtension/ToolWindows/ThemeWindow/ThemeWindowControl.xaml

@@ -24,6 +24,7 @@
                 <ColumnDefinition Width="Auto"/>
                 <ColumnDefinition Width="*"/>
                 <ColumnDefinition Width="Auto"/>
+                <ColumnDefinition Width="Auto"/>
             </Grid.ColumnDefinitions>
 
             <StackPanel 
@@ -56,6 +57,14 @@
                 Content="Open Dialog" 
                 Command="{Binding OpenDialogCommand}" 
                 VerticalAlignment="Center"
+                Margin="10,0"
+                />
+
+            <Button 
+                Grid.Column="3"
+                Content="Customized Styles" 
+                Command="{Binding OpenCustomStylesCommand}" 
+                VerticalAlignment="Center"
                 />
         </Grid>
 

demo/VSSDK.TestExtension/ToolWindows/ThemeWindow/ThemeWindowControlViewModel.cs

@@ -64,6 +64,7 @@ public bool UseVsTheme
 
 #pragma warning disable VSTHRD012 // Provide JoinableTaskFactory where allowed
         public ICommand OpenDialogCommand => new DelegateCommand(() => OpenDialog());
+        public ICommand OpenCustomStylesCommand => new DelegateCommand(() => OpenCustomStyles());
 #pragma warning restore VSTHRD012 // Provide JoinableTaskFactory where allowed
 
         private void OpenDialog()
@@ -72,6 +73,12 @@ private void OpenDialog()
             dialog.ShowModal();
         }
 
+        private void OpenCustomStyles()
+        {
+            CustomizedStylesDialog dialog = new CustomizedStylesDialog();
+            dialog.ShowModal();
+        }
+
         private async Task ApplyThemeAsync(Theme theme)
         {
             // There doesn't appear to be a way to set the theme programatically, 

demo/VSSDK.TestExtension/VSSDK.TestExtension.csproj

@@ -79,6 +79,9 @@
       <DependentUpon>RunnerWindowControl.xaml</DependentUpon>
     </Compile>
     <Compile Include="ToolWindows\RunnerWindowMessenger.cs" />
+    <Compile Include="ToolWindows\ThemeWindow\CustomizedStylesDialog.xaml.cs">
+      <DependentUpon>CustomizedStylesDialog.xaml</DependentUpon>
+    </Compile>
     <Compile Include="ToolWindows\ThemeWindow\ThemedControl.cs" />
     <Compile Include="ToolWindows\ThemeWindow\ThemeWindow.cs" />
     <Compile Include="ToolWindows\ThemeWindow\ThemeWindowControl.xaml.cs">
@@ -141,6 +144,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
     </Page>
+    <Page Include="ToolWindows\ThemeWindow\CustomizedStylesDialog.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
     <Page Include="ToolWindows\ThemeWindow\ThemedControl.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>