Normally you register the editor CSS thru the web configuration:
<?xml version="1.0" encoding="utf-8"?> <configuration> <episerver> <applicationSettings uiEditorCssPaths=”/Content/bootstrap.css, /Content/site.css” /> </episerver> </configuration>
But sometimes in scenarios where you are using a optimization library like Combres to pack the CSS:s you’ll need to set the CSS programmatically. If you have a multisite scenario you’ll need different CSS for each site. Thanks to other blogposts I’ve made a variation of a descriptor that works for my solution.
Since I use bootstrap.css I had to put some classes like “container” in the tinyMCE editors body to make bootstrap work. I also put the site name in the body class and the property name. Finally I add a tinymce.css where I can put custom editor css and editor styles, 
with help of EditMenuTitle and EditMenuName (which won’t work if you don’t have a stop “;”)
Because the WYSIWYG uses an iframe, be aware of the column widths will not work, since the editor width is smaller.
namespace EPiServerXhtmlStringSettings.Business.EditorDescriptors
{
[EditorDescriptorRegistration(TargetType = typeof(XhtmlString), EditorDescriptorBehavior = EditorDescriptorBehavior.PlaceLast)]
public class CustomXhtmlStringEditorDescriptor : EditorDescriptor
{
public override void ModifyMetadata(EPiServer.Shell.ObjectEditing.ExtendedMetadata metadata,
IEnumerable<Attribute> attributes)
{
if (metadata.CustomEditorSettings.ContainsKey("uiParams")) {
object data = metadata.CustomEditorSettings["uiParams"];
var dictionary = (Dictionary<string, object>)data.GetType().GetProperty("inlineSettings").GetValue(data, null);
dictionary["body_class"] = dictionary["body_class"] + " " + metadata.PropertyName + " PageText container contentpage " + EPiServer.Web.SiteDefinition.Current.Name;
}
if (SiteDefinition.Current.Name.ToLower().Equals("name1"))
EPiServer.Configuration.Settings.Instance.UIEditorCssPaths = "/content/bootstrap.min.css, " + Combres.WebExtensions.CombresUrl("Name1SiteCss") + "fake.css, /Templates/Public/Styles/Common/tinymce.css";
else if (SiteDefinition.Current.Name.ToLower().Equals("name2"))
EPiServer.Configuration.Settings.Instance.UIEditorCssPaths = "/content/bootstrap.min.css, " + Combres.WebExtensions.CombresUrl("Name2SiteCss") + "fake.css, /Templates/Public/Styles/Common/tinymce.css";
else
EPiServer.Configuration.Settings.Instance.UIEditorCssPaths = "/content/bootstrap.min.css, " + Combres.WebExtensions.CombresUrl("DefaultSiteCss") + "fake.css, /Templates/Public/Styles/Common/tinymce.css";
}
}
}
Because I want to use this custom change globally i do use EditorDescriptorBehavior.PlaceLast and inherit EditorDescriptor.
Limitations in UIEditorCssPaths: You can’t have absolute urls nor non file urls (eg not without a file extention)
More reading: