In a project I'm working on I used settings transforms in code to handle multi site content css and style formats. Here is a little example of that:

[ModuleDependency(typeof(TinyMceInitialization))]
public class SiteTinyMceInitialization : IConfigurableModule
{
    public void ConfigureContainer(ServiceConfigurationContext context)
    {
        context.Services.Configure<TinyMceConfiguration>(config =>
        {
            var locator = ServiceLocator.Current;

            config.Default()
                .AddSettingsTransform("default-content-css-transform", (settings, content, propertyName) =>
                {
                    var contentCss = $"/public/tinymce-{SiteDefinition.Current.Name}.css";

                    settings.ContentCss(contentCss);
                })
                .AddPlugin("anchor table code")
                .Toolbar(
                    "epi-link anchor | image epi-image-editor | bold italic | alignleft aligncenter alignright | " +
                    "styleselect | table | cut copy pastetext paste removeformat fullscreen | bullist numlist | " +
                    "subscript superscript | undo redo find code"
                )
                .AddSettingsTransform("default-style-select-transform", (settings, content, propertyName) =>
                {
                    settings.StyleFormats(DefaultStyleFormats[SiteDefinition.Current.Name]);
                });
        });
    }

    public void Initialize(InitializationEngine context)
    {
    }

    public void Uninitialize(InitializationEngine context)
    {
    }

    /// <summary>
    /// HappyME style select formats
    /// </summary>
    internal static object[] HappyMeDefaultStyleFormats = 
    {
        new
        {
            title = "Heading",
            items = new[]
            {
                new {title = "Heading 1", format = "h1"},
                new {title = "Heading 2", format = "h2"},
                new {title = "Heading 3", format = "h3"},
                new {title = "Heading 4", format = "h4"}
            }
        },
        new
        {
            title = "Text size",
            items = new[]
            {
                new {title = "Normal", inline = "span", classes = "size--normal"},
                new {title = "Large", inline = "span", classes = "size--large"}
            }
        },
        new
        {
            title = "Font weight",
            items = new[]
            {
                new {title = "Light", inline = "span", classes = "weight--light"},
                new {title = "Regular", inline = "span", classes = "weight--regular"},
                new {title = "Semi Bold", inline = "span", classes = "weight--semi-bold"},
                new {title = "Bold", inline = "span", classes = "weight--bold"},
                new {title = "Extra Bold", inline = "span", classes = "weight--extra-bold"}
            }
        },
        new
        {
            title = "Color",
            items = new[]
            {
                new {title = "Black", inline = "span", classes = "color--black"},
                new {title = "Dark grey", inline = "span", classes = "color--dark-gray"},
                new {title = "Grey", inline = "span", classes = "color--gray"},
                new {title = "Brand", inline = "span", classes = "color--brand"}
            }
        },
        new
        {
            title = "Background color",
            items = new[]
            {
                new {title = "Blue", inline = "span", classes = "bg--blue"},
                new {title = "Green", inline = "span", classes = "bg--green"},
                new {title = "Orange", inline = "span", classes = "bg--orange"},
                new {title = "Pink", inline = "span", classes = "bg--pink"}
            }
        }
    };

    /// <summary>
    /// OSL default style select formats
    /// </summary>
    internal static object[] OslDefaultStyleFormats =
    {
        new
        {
            title = "Heading",
            items = new[]
            {
                new {title = "Heading 1", format = "h1"},
                new {title = "Heading 2", format = "h2"},
                new {title = "Heading 3", format = "h3"},
                new {title = "Heading 4", format = "h4"}
            }
        },
        new
        {
            title = "Text size",
            items = new[]
            {
                new {title = "Normal", inline = "span", classes = "size--normal"},
                new {title = "Small", inline = "span", classes = "size--small"},
                new {title = "Large", inline = "span", classes = "size--large"}
            }
        },
        new
        {
            title = "Font weight",
            items = new[]
            {
                new {title = "Extra Light", inline = "span", classes = "weight--extra-light"},
                new {title = "Light", inline = "span", classes = "weight--light"},
                new {title = "Regular", inline = "span", classes = "weight--regular"},
                new {title = "Medium", inline = "span", classes = "weight--medium"},
                new {title = "Semi Bold", inline = "span", classes = "weight--semi-bold"}
            }
        },
        new
        {
            title = "Indent",
            items = new[]
            {
                new {title = "Small", inline = "span", classes = "indent--small"},
                new {title = "Large", inline = "span", classes = "indent--large"}
            }
        }
    };

    /// <summary>
    /// VitaePro default style select formats
    /// </summary>
    internal static object[] VitaeProDefaultStyleFormats =
    {
        new
        {
            title = "Heading",
            items = new[]
            {
                new {title = "Heading 1", format = "h1"},
                new {title = "Heading 2", format = "h2"},
                new {title = "Heading 3", format = "h3"},
                new {title = "Heading 4", format = "h4"}
            }
        },
        new
        {
            title = "Text size",
            items = new[]
            {
                new {title = "Normal", inline = "span", classes = "size--normal"},
                new {title = "Medium", inline = "span", classes = "size--medium"},
                new {title = "Large", inline = "span", classes = "size--large"}
            }
        },
        new
        {
            title = "Font weight",
            items = new[]
            {
                new {title = "Regular", inline = "span", classes = "weight--regular"},
                new {title = "Bold", inline = "span", classes = "weight--bold"}
            }
        },
        new
        {
            title = "Color",
            items = new[]
            {
                new {title = "Black", inline = "span", classes = "color--black"},
                new {title = "Grey", inline = "span", classes = "color--gray"},
                new {title = "Brand", inline = "span", classes = "color--brand"}
            }
        },
        new
        {
            title = "Other",
            items = new[]
            {
                new {title = "Question box", inline = "", block = "p", classes = "info-box"},
                new {title = "Border bottom", inline = "span", block = "p", classes = "border--bottom"}
            }
        }
    };

    public static IDictionary<string, object[]> DefaultStyleFormats = new Dictionary<string, object[]>
    {
        { SiteNames.HappyME, HappyMeDefaultStyleFormats },
        { SiteNames.OSL, OslDefaultStyleFormats },
        { SiteNames.VitaePro, VitaeProDefaultStyleFormats }
    };
}

Interesting, I didn't realize you could use the site context that way. I thought that it would be resolved one time at app startup and that was it. I'll give this a shot, thanks

Yes, it's easy to assume that. :) The settings transforms are registered during app startup but they are applied when a tinymce editor is being instantiated in edit mode (when you do have a site context).

Has anyone found a way to vary ContentCss on something more dynamic? My use case right now is what the old Dynamic Property setting solved. I have a branch of pages that have other typography but the content types are shared with the rest of the site.

Wouldn't that be possible with a settings transform as well? Something like this:

public interface ITinyMceContentCssPathSetting : IContent
{
    string ContentCssPath { get; }
}

.AddSettingsTransform("dynamic-content-css", (settings, content, propertyName) => 
{
    var closestContentCssSetting = contentLoader.GetClosestContent<ITinyMceContentCssPathSetting>(content);

    if (!string.IsNullOrEmpty(closestContentCssSetting?.ContentCssPath))
    {
        settings.ContentCss(closestContentCssSetting.ContentCssPath);
    }
})

public static T GetClosestContent<T>(this IContentLoader contentLoader, IContent content)
{
    if (content is T)
    {
        return (T)content;
    }

    var current = contentLoader.Get<IContent>(content.ParentLink);

    while (!current.ContentLink.CompareToIgnoreWorkID(ContentReference.RootPage))
    {
        if (current is T)
        {
            return (T)current;
        }

        current = contentLoader.Get<IContent>(current.ParentLink);
    }

    return default(T);
}

EDIT: It would probably be a better idea to write a method that finds the closest content that implements ITinyMceContentCssPathSetting AND has a value for ContentCssPath property. :)

Of course! I was a bit stuck in my thinking and missed the Content part of the first example. Very easy to get what I needed...

Are the keys you have in your examples existing ones or just new strings?

The documentation part where AddSettingsTransform is handled for reference:
https://world.episerver.com/documentation/developer-guides/CMS/add-ons/customizing-the-tinymce-editor-v2/configuration-api/

The keys are just new strings. :)

Cool. I wonder if there's a case for knowing and replacing an OOTB key's value? Docs: "You can also remove registered transformations if you know the key."

Oh, they don’t reveal the keys? ;) I guess you have to put your Sherlock coat on and fire up dotPeek or similar.

It's always on and I will!