Update README.md

Added an example of how to go downlevel with the exported editor markup.

README.md

@@ -25,6 +25,70 @@ Using the tool
 ---
 If you wish to get access to the source code without using Git you can download the zip file directly by clicking on the "Clone or Download" dropdown button at the top right of the repo landing page and selecting the last option labeled "Download Zip".
 
+Theming for Downlevel
+---
+Although the API used in the exported code for this tool is for version 17744 or greater, it's not too complicated to get your theme to work on earlier SDK versions.
+
+When you export your theme, you'll see a ResourceDictionary markup with a ColorPaletteResources definition similar to this:
+
+```xaml
+<ResourceDictionary.ThemeDictionaries>
+    <ResourceDictionary x:Key="Default">
+        <ResourceDictionary.MergedDictionaries>
+            <ColorPaletteResources Accent="#FF8961CC" AltHigh="#FF000000" AltLow="#FF000000" AltMedium="#FF000000" AltMediumHigh="#FF000000" AltMediumLow="#FF000000" BaseHigh="#FFFFFFFF" BaseLow="#FF64576B" BaseMedium="#FFB6AABC" BaseMediumHigh="#FFCBBFD0" BaseMediumLow="#FF8D8193" ChromeAltLow="#FFCBBFD0" ChromeBlackHigh="#FF000000" ChromeBlackLow="#FFCBBFD0" ChromeBlackMedium="#FF000000" ChromeBlackMediumLow="#FF000000" ChromeDisabledHigh="#FF64576B" ChromeDisabledLow="#FFB6AABC" ChromeGray="#FFA295A8" ChromeHigh="#FFA295A8" ChromeLow="#FF332041" ChromeMedium="#FF3F2E4B" ChromeMediumLow="#FF584960" ChromeWhite="#FFFFFFFF" ListLow="#FF3F2E4B" ListMedium="#FF64576B" />
+        <ResourceDictionary>
+            <Color x:Key="RegionColor">#FF262738</Color>
+            <SolidColorBrush x:Key="RegionBrush" Color="{StaticResource RegionColor}" />
+        </ResourceDictionary>
+    </ResourceDictionary.MergedDictionaries>
+</ResourceDictionary>
+```
+
+ColorPaletteResources is a friendly API for our SystemColors that sit within generic.xaml and allows for those SystemColors to be scoped at any level. 
+
+If you wanted to enable this same theme to work downlevel, you would have to define each SystemColor individually with each color from your theme:
+
+```xaml
+<ResourceDictionary>
+    <ResourceDictionary.ThemeDictionaries>
+        <ResourceDictionary x:Key="Default">
+            <Color x:Key="SystemAltHighColor">#FF000000</Color>
+            <Color x:Key="SystemAltLowColor">#FF000000</Color>
+            <Color x:Key="SystemAltMediumColor">#FF000000</Color>
+            <Color x:Key="SystemAltMediumHighColor">#FF000000</Color>
+            <Color x:Key="SystemAltMediumLowColor">#FF000000</Color>
+            <Color x:Key="SystemBaseHighColor">#FFFFFFFF</Color>
+            <Color x:Key="SystemBaseLowColor">#FF64576B</Color>
+            <Color x:Key="SystemBaseMediumColor">#FFB6AABC</Color>
+            <Color x:Key="SystemBaseMediumHighColor">#FFCBBFD0</Color>
+            <Color x:Key="SystemBaseMediumLowColor">#FF8D8193</Color>
+            <Color x:Key="SystemChromeAltLowColor">#FFCBBFD0</Color>
+            <Color x:Key="SystemChromeBlackHighColor">#FF000000</Color>
+            <Color x:Key="SystemChromeBlackLowColor">#FFCBBFD0</Color>
+            <Color x:Key="SystemChromeBlackMediumLowColor">#FF000000</Color>
+            <Color x:Key="SystemChromeBlackMediumColor">#FF000000</Color>
+            <Color x:Key="SystemChromeDisabledHighColor">#FF64576B</Color>
+            <Color x:Key="SystemChromeDisabledLowColor">#FFB6AABC</Color>
+            <Color x:Key="SystemChromeHighColor">#FFA295A8</Color>
+            <Color x:Key="SystemChromeLowColor">#FF332041</Color>
+            <Color x:Key="SystemChromeMediumColor">#FF3F2E4B</Color>
+            <Color x:Key="SystemChromeMediumLowColor">#FF584960</Color>
+            <Color x:Key="SystemChromeWhiteColor">#FFFFFFFF</Color>
+            <Color x:Key="SystemChromeGrayColor">#FFA295A8</Color>
+            <Color x:Key="SystemListLowColor">#FF3F2E4B</Color>
+            <Color x:Key="SystemListMediumColor">#FF64576B</Color>                
+            <Color x:Key="SystemAccentColor">#FF8961CC</Color>
+            
+            <Color x:Key="RegionColor">#FF262738</Color>
+            <SolidColorBrush x:Key="RegionBrush" Color="{StaticResource RegionColor}"/>
+         </ResourceDictionary>
+    </ResourceDictionary.ThemeDictionaries>
+</ResourceDictionary>
+```
+In this case we're using the Lavendar theme to show this downlevel transition.
+
+> **Warning:** Although this markup format change will enable your theme to be applied across controls in earlier SDK versions, it **will not** work on a page, container or control scoped level. ColorPaletteResources is the API that allows **scoping behavior**. This markup format will only work at the app.xaml level for earlier SDKs.
+
 Contributions
 ---
 This tool was created directly from the feature team and although we welcome your input and suggestions for improvements to the tool, we are not accepting any features or changes from the public at this time. Please check back regularly as we may evolve our contribution model in the future.