Is there a way to waste less space in the leading column?

I recently upgraded to OF2, and I’m trying to adjust to the changes. One thing I really wish I could do is make it use less horizontal space. Is there any way to adjust the space between the sidebar and the first-level items in a project? By this I mean the blank space indicated by the red arrow in this image:

I’ve found the theme editor posted somewhere else in these forums, and that’s been useful to adjust some color issues, but it doesn’t look like it provides a way to adjust the width.

Is there a way to do it?