I had a similar problem. To modify the width, I had to change the width value for using the classes used by Mendix.
First make sure in your layout, the sidebar has then class “region-sidebar”:
I then created a width variable to store the width when the navbar is closed:
I had to then use the chain of classes that resulted in the change of width:
This then results in the navbar width being changed in its closed state:
I hope there’s an easier way of changing the width of the closed sidebar, but this was the only way I found that works.
Hope this helps,