while upgrading from 2.3.3 to 2.3.4 I have noticed that my custom editor.css won't load anymore for TinyMCE in the CMS. So I have traced the problem and I found out that you moved HtmlEditorConfig::get('cms')->setOptions() from LeftAndMain to cms/_config. So the problem is that you actually check SSViewer::current_theme() before I call SSViewer::set_theme().
My solution so far is to simply set the content_css value again in my own _config but I guess this shouldn't be the point.
'content_css'=>'cms/css/editor.css, '.(SSViewer::current_theme() ? THEMES_DIR . "/" . SSViewer::current_theme() : project()) . "/css/editor.css"
But maybe I overlooked something.