We would like to consult with you regarding CSS/SASS customization with ExtJS.

We're working on an advanced application based on ExtJS framework and now we're starting to work on the design and graphics.
As we see it there are 2 options to customize the look&feel of our application:
1. Take Ext's SASS code and customize it - this will cause upgrades problem...
2. Take Ext's SASS code and extend it with our customized CSS.

We would like to here your opinion? What is the recommended way to do it?

We see there are config parameters we can change but it's not flexible for us.