In Ubuntu with the dark theme Ambiance + gtk 3, the titlebar, toolbar, context menus, and dialog boxes are all rendered black on white text. However the sidebar is white on black. Other distros like Debian and Fedora with a dark theme + gtk3 display everything black on white text, including the sidebar.
Ambiance is not a dark theme. It’s a light theme which happens to use a dark color for styling primary toolbars.
You can suggest a design change by filing a question or a bug report at https://launchpad.net/ubuntu-themes
Should a launchpad bug be filed under ubuntu-themes or libreoffice?
(In reply to Luke from comment #2)
> Should a launchpad bug be filed under ubuntu-themes or libreoffice?
Both (on one bug -- launchpad allows that).