Catfish allows you to adjust and tweak almost all areas of the editor using a simple form to adjust colours, words, and borders.
Visit Admin -> Catfish -> Settings -> Editor Settings
The list is extensive, but most labels should be relatively self explanatory in terms of what they do to the editor window.
Please bear in mind design changes may be cached by the browser and therefore require to clear session cache or try on a separate browser to see any changes
If you have any questions about this, please raise a ticket.