I don't know of such a setting in Fluxbox. Maybe someone else does and will enlighten both of us.
You can approximate it in Fluxbox Settings-->Tabs by enabling taps on the edge of the application window nearest the toolbar.
For example, I have my toolbar on the top right, 67% width, autohide off, and application tabs top left. If I maximize an app, the tab is part of the maximization, so it hits the top of the screen at the top left, blocking the rest of the window from covering the taskbar.
I can still, however, move an application window so that it covers the taskbar. See the screenshot.
Such a setting, if you find one, would be better in your ~/.fluxbox/init file than in an individual theme file.
More about Flux in the Fluxbox Wiki: