Level Pages Appear the Same as Level 2 Pages

I'm using the Flexipages plugin to display only my subpages in the sidebar. I’m not displaying primary pages in the side navigation pane, because they are displayed across the top of the page. One of my subpages (History) has its own subpages (X,Y,and Z), which don’t appear until I’ve selected the History page. When I click the History page, the subpages do appear, except they display at the top of the side navigation panel at the same level as their parent page, rather than below their parent, the History page, and indented.

I checked with the Flexipages developer, and he said that this issue is related to the theme and styling. Any suggestions?

Thanks,
Michael