Found the solution, answering my own question:

public class DefaultTinyMceSettings : PropertySettings<TinyMCESettings> 
{
        public override TinyMCESettings GetPropertySettings()
        {
            var settings = new TinyMCESettings();

            //much code removed, such small, wow

            settings.ContentCss = "~/static/css/editor.css";  // new line of code

            return settings;
        }
}

we already had an DefaultTinyMceSettings class, what I've added (with an example above) is the line about ContentCss. Thats fixed my issue. That css file defines all the custom TinyMCE styles that are in use.

[PropertySettings(typeof(Models.Properties.DefaultTinyMceSettings))]  // new line fixes the issue      
public virtual XhtmlString Body { get; set; }