Phil, 9-22-2013 (RW 6.4, Qt 4.8.5).

Note: Not sure this is right. Incorrect ones seem to default to Windows.
This might be the case for the "other" windows styles, gtk and macintosh.

Plastique, Cleanlooks, Cde and Motif do work, and all support:
 (1) visible splitter controls,
 (2) distinct appearance of RADIO buttons in menus.

riverware.exe -style Plastique
riverware.exe -style GTK
riverware.exe -style Cleanlooks
riverware.exe -style WindowsXP
riverware.exe -style WindowsVista
riverware.exe -style Windows
riverware.exe -style Macintosh
riverware.exe -style CDE
riverware.exe -style Motif

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