TinyMCE has a plugin called "advhr" - see http://www.tinymce.com/wiki.php/Plugin3x:advhr
and for example this post explains how to use it with moodle 2.2.4:
https://moodle.org/mod/forum/discuss.php?d=208177#p908181
In theory you should be able to add any classes both to editor.css and your theme css files and if those elements support the properties given by your custom css you should see the result immediately. One possible reason for failure might be that you may need to purge cache (or switch temporarely to theme designer mode) to get the correct css to cached css.
And note that in your case it should be
.hr
{
color: #4f81bd;
max-width:90;
}
in a css file if you want to give class "hr" to some (any) element
You can test advhr-plugin 
in http://www.tinymce.com/tryit/3_x/full.php