How to change font family and size of Safari's web inspector on El Capitan
1.Disable System Integrity Protection(SIP)
Reboot your Mac holding down "cmd + R" keys to boot into Recovery Mode.
Launch Terminal in Utilities menu on menu bar.
Type
csrutil disable
to disable SIP.
Reboot your Mac.
2.Copy "Main.css" file at
/System/Library/PrivateFrameworks/WebInspectorUI.framework/Versions/A/Resources
to your Desktop and make back up the file.
3.Open "Main.css" file with text editor and search "cm-s-default,.syntax-highlighted".
Then you'll find "{color:black;font-family:Menlo,monospace;font-size:11px}".
Change font-family and font-size values to what you want and save the changes.
4.Revert the changed "Main.css" file to original directory.
5.Enable SIP
Reboot your Mac holding down "cmd + R" keys to boot into Recovery Mode.
Launch Terminal in Utilities menu on menu bar.
Type
csrutil enable
to enable SIP.
Reboot your Mac again.
*At your OWN risk!! if you do this.