Adding markers around modern support

Author: VesaJuvonen

docs/solution-guidance/SharePoint-site-branding-and-page-customization-solutions.md

@@ -13,6 +13,9 @@ You can customize the look and feel of a SharePoint site in two ways:
 
 - **By creating custom cascading style sheets (CSS) to apply to SharePoint Online sites**. You can use a SharePoint Add-in and the remote provisioning pattern to provision SharePoint sites to use custom CSS.
 
+> [!IMPORTANT] 
+> These extensibility options are **only** available for classic SharePoint experiences. You cannot use these options with modern experiences in SharePoint Online, like with communication sites.
+
 Branding changes range from low-cost and simple to high-cost and complex. Users can use the UI to apply composed looks, which include a background image, color palette, fonts, a master page, and an associated .preview file for the master page. You can use the SharePoint theming engine to design composed looks and provision sites, and you can create custom CSS to modify the look and feel of your site and its elements.
 
 > [!IMPORTANT] 

docs/solution-guidance/Use-CSS-to-brand-pages.md

@@ -1,14 +1,17 @@
 ---
 title: Use CSS to brand SharePoint pages
 description: Use CSS to support branding and site design in SharePoint. Find out about CSS master pages, the corev15.css file, and composed looks in custom branding.
-ms.date: 4/25/2018
+ms.date: 4/12/2019
 localization_priority: Priority
 ---
 
 # Use CSS to brand SharePoint pages
 
 Cascading style sheet (CSS) plays a large role in SharePoint branding. To successfully customize the site design in SharePoint and SharePoint Online, it's useful to be familiar with how SharePoint uses CSS.
 
+> [!IMPORTANT] 
+> This extensibility option is **only** available for classic SharePoint experiences. You cannot use this option with modern experiences in SharePoint Online, like with communication sites.
+
 ## CSS branding overview
 
 When you create a SharePoint site collection, SharePoint creates a Master Page Gallery (_catalogs/masterpage) where most branding assets, including .master, .css, image, HTML, and JavaScript files are stored.

docs/solution-guidance/Use-composed-looks-to-brand-SharePoint-sites.md

@@ -9,6 +9,9 @@ localization_priority: Priority
 
 **Composed looks** are out-of-the-box themes that are included in SharePoint and SharePoint Online. Apply composed looks, including colors, fonts, and a background image, to your SharePoint and SharePoint Online sites by using the SharePoint theming engine.
 
+> [!IMPORTANT] 
+> This extensibility option is **only** available for classic SharePoint experiences. You cannot use this option with modern experiences in SharePoint Online, like with communication sites.
+
 To apply a composed look to a SharePoint site, select **Site Settings** > **Look and Feel** > **Change the look**. You can then use the Change the Look Wizard to customize the colors, fonts, master page, and background image of a composed look. The Change the Look Wizard copies, transforms, and stores CSS in SharePoint's content database. It also recolors images and stores them in the content database. 
 
 ## SharePoint theming engine

docs/solution-guidance/Use-remote-provisioning-to-brand-SharePoint-pages.md

@@ -11,6 +11,9 @@ You can apply and interact with themes by using remote provisioning features in
 
 -  [ApplyTheme method](http://msdn.microsoft.com/library/52c567e8-03e6-7ba3-a9ed-cf4e3c22dbdd%28Office.15%29.aspx)
 -  [ThemeInfo class](https://msdn.microsoft.com/en-us/library/office/microsoft.sharepoint.client.themeinfo.aspx)
+
+> [!IMPORTANT] 
+> This extensibility option is **only** available for classic SharePoint experiences. You cannot use this option with modern experiences in SharePoint Online, like with communication sites.
     
 The **ApplyTheme** method powers the Change the Look Wizard. The wizard applies a composed look, or a custom look, to a SharePoint site by using specified components. Themes are applied on a site-by-site basis.
 

docs/solution-guidance/update-the-branding-of-existing-sharepoint-sites-and-page-regions.md

@@ -1,14 +1,18 @@
 ---
 title: Update the branding of existing SharePoint sites and page regions
 description: Customize and then refresh the branding of existing SharePoint sites or regions of SharePoint pages, including the ribbon, the site navigation, the Settings menu, the tree view, and the page content.
-ms.date: 4/25/2018
+ms.date: 4/12/2019
 localization_priority: Normal
 ---
 
 # Update the branding of existing SharePoint sites and page regions
 
 You can refresh the branding on your existing SharePoint sites and site collections, as well as customize regions of SharePoint pages. You might want to do this, for example, when you update to a new version of the site. 
 
+> [!IMPORTANT] 
+> This extensibility option is **only** available for classic SharePoint experiences. You cannot use this option with modern experiences in SharePoint Online, like with communication sites.
+
+
 ## Refresh branding of existing sites and subsites
 
 The [Branding.Refresh](https://github.com/SharePoint/PnP/tree/dev/Samples/Branding.Refresh) sample in the Office 365 Developer Patterns and Practices project on GitHub shows you how to use the OfficeDevPnP.Core library to iterate over existing sites and subsites to verify and update the applied branding. The sample shows how to update site branding, but the same concept can be used to do other things, such as deploy a new library to a list of sites, or update a custom action that was deployed during provisioning. You can use the same process to move existing sites to a newer version.