Sorry, now I have seen that this was already written.
You have to turn off accessibility mode, because the fallback for this mode is the standard signature theme. It has something to do with the color contrast. I don't know if there exists a sap note for this, but I checked that and with no acc mode the colors are rendered correctly.
By the way this is also important in NWBC 4.0 and 5.0 if you don't want to use the SAP GUI Theme. NWBC tries to start SAP GUI in corbu theme, but if you have activated acc mode, then the theme is always standard signature theme.
Best Regards,
Thomas