When viewing drainage using Drainage > View > Drainage, the styles assigned to the structures in the structures.dat file are not honored.
The way in which drainage is displayed is dependent on the Style Lock: Tools > Locks > Style.
If the style lock is on, the symbology for structures is defined by the Style assigned to each individual structure and can be edited with the Drainage > Edit/Review > Styles Tab. The display will automatically process when selecting the Drainage > View > Drainage option without the command dialog opening because it is using the styles assigned to each individual structure for the display.
If the style lock is off, the symbology for structures is defined by the properties assigned to each object listed for display under Drainage > View > Drainage.