xfa/fwl/theme contains code for rendering XFA widgets.
TP stands for Theme Part.
CFWL_WidgetTP contains much of the code common to more than one widget.
The other CFWL_TP classes derive from it and know how to draw the pieces specific to their respective widget.
The inheritance hierarchy for this directory is:
All these widget TP classes are composed into CXFA_FWLTheme, which implements IFWL_ThemeProvider (and is the only class that does). CXFA_FWLTheme receives DrawBackground() calls from CFWL widgets to draw themselves and routes them to the TP (Theme Part) corresponding to that widget.