The Issue With Customise:
Wouldn't it be nice if WordPress made the Customise area shown in the below image resizable?
As it is at the moment, it can be a little tricky working in a squashed sidebar.
Because of this I usually do all my CSS in an external editor then copy it into the sidebar area.
It's Under Discussion at WordPress
The WordPress devs were looking at improving the Customizer UI, which you can see here. This would solve the issue, but until then...
This Question is Not About CSS
This question isn't about why I've used the Customizer for CSS - I have my reasons - That's another topic altogether.
What is the best way of customising this WordPress area to make it expandable or wider without the need for a plugin? This would make it easier to work with all customizations, not just CSS.
*This change must be part of the child theme to prevent being overwritten by updates.