Editing Additional CSS

Hello,

Wordpress 4.7 has introduced ‘Additional CSS’, accessible via Appearance -> Customize.

The problem I have is that editing that is a nightmare. It’s just a narrow strip on the left.

Is there a better way to edit the “Additional CSS” (preferably ‘full screen and with syntax highlighting)

Alternatively, is there a (simple) way to upload an already existing custom CSS — without copy&paste ?

Thank you.