Currently you can right-click the bookmarks and choose "Collapse All"
If there's only one root bookmark (as is the case for documents exported from a wordprocessor with bookmarks generated from the headings) this isn't much use as you're going to use it followed by expanding it; for such cases it would make sense if it automatically expanded the root one level
Secondly, the menu is very short, it may be useful to add a few explicit collapse levels e.g. "Show to level 1", "Show to level 2"