Is it possible to show custom Wiki sidebar as compact as default one?


I like how default Wiki sidebar is shown.
But in my case there is deep nested tree of Wiki pages and I don’t want to show all of them on sidebar.
I created custom Wiki sidebar using markdown’s unsorted list, but it does not look so compact and slim as default. I also tried to use pure html (ul tag), but I couldn’t apply any styles on it.
Is there any way to show custom Sidebar as default one?

Thank you!