I was looking at one of my older wikis today where I had quickly hacked together a menu/sidebar on the left of the default theme. Since there has been discussion around this recently, I have extracted the salient bits for anyone that wants to tinker with it: TiddlyWiki — a non-linear personal web notebook
Consider it a potential starting point rather than a finished product.
As I recall, the mobile/small screen presentation needs work and currently the left menu just gets hidden on smaller screens.