A while ago I put up the same kind of request, coming from Textpattern I was used to have a tab for Stylesheets. I found using the file manager in MODx was annoying going through the folder structure each time (assets\templates\....). Plus, I’d dream of syntax highlighting (though it would be sugar coating here...).
In the meantime, I have learn to use the "stylesheet as document" method (e.g create a new document with
text/css as content-type and
_blank as template, then link it this way :
<link rel="stylesheet" type="text/css" href="[~id~]" />, with id being of course the id of your css document).
The benefit of this method is that MODx parses your CSS : this means you can use... chunks in your stylesheet, and it’s a very powerful method to build modular css
The other benefit is it’s easier to work with your CSS, since you edit them either directly as a document or like me, you edit the css chunks you have created.
I use it ever since !
More detail here :
http://modxcms.com/forums/index.php/topic,2343.0.html
http://modxcms.com/forums/index.php/topic,3068.0.html
http://modxcms.com/forums/index.php/topic,3052.0.html