by Tim Hunt.
Do you mean the style of the text and other content that appears in TinyMCE editor? If so, you can control this in your theme.
In theme/yourtheme/config.php, add the line
$THEME->editor_sheets = array('editor');
Then, create a file theme/yourtheme/styles/editor.css. Any CSS you put in there will be applied to the content in Tiny MCE editors.
If I have misunderstood, and instead you want to be able to configure the TinyMCE toolbars, then that feature was added in Moodle 2.4.