Hi there,
You need to look in the plugin in the back end. Goto plugin manager and look for Editor – TinyMCE
In there there is a parameter ‘Use template Css‘:
– If it’s set to ‘Yes‘, by default, the Plugin looks for an editor.css file, if it cannot find one in default template, it will load the editor.css file in system template. So in this case you can override the css file in your default template.
– Regarding the case that you wanna use your own css file, you should set parameter ‘Use template Css‘ to ‘No‘. Then in parameter ‘Custom CSS Classes‘, you fill with url to your custom css file.
Hope this helps.