Hi Christian,
The cell is specified in the Element Templates contained in GridTemplate.dgnlib.
The DGNLIB containing the Element Templates can be specified by BB_GRID_TEMPLATE_DGNLIB, customising them is described in more detail in Customising Grid Systems Symbology With Element Templates in SS6.
Other settings are available from the Settings button in Floor Manager, changes to those settings are stored in the BB_FloorMaster.dgnlib of the active project.