Extending backend UI in Ucommerce 9
A word on Kondo UI
In version 9.0 Ucommerce ships with a new UI stack built on Vue.js
In favor of releasing faster and establishing knowledge on how the UI behaves, extensibility has been prioritized for a later version.
This means that you're not able to change how the new UI stack behaves or change the look and feel. If you're still interested in changing the UI this is still possible with the webforms UI stack and the following articles provides knowledge how to.