I just upgraded a couple sites to 2.3.4 (and have the same problem on all of them, including one fresh install at 2.3.4) and editor.css is no longer getting loaded by TinyMCE. I traced my requests with Firebug and saw that it was trying to call it at '[site_root]/css/editor.css' instead of from my theme directory. Looking through the code for a bit, it looks like it should prepend the 'THEMES_DIR' variable in front, but for whatever reason that's loading in blank. All these sites worked in 2.3.3.
The code is in the _config.php file for the cms module. This is the line is question:
NB: If I hard code the file to point to my editor.css file in the theme directory, it works, so I know it's not a problem with reading the file. Also, all the other function calls within the _config.php file work.
I too had this problem on a fresh 2.3.4 install which was not there in a 2.3.1. I checked the code in the _config.php of the cms module as referred to above and added a print_r() of the content_css line.
It was incorrect and contained neither the theme editor.css or the alternative project editor.css
After that I tried a print_r() of just SSViewer::current_theme() and it was blank which was obviously wrong
just above the HtmlEditorConfig::get('cms')->setOptions()
did the trick
This means that when this cms config is loaded the site config has not yet been loaded (which is clearly incorrect in this context and is a bug)
freakout, you should still be able to use the second workaround JayDevlin suggested, reseting the content_cms in 'mysite/_config.php', since that should be different for each site (though conveniently you can paste the exact same code in each one).