AFAIK, the shadow can't be modified by the user. There are no settings on the Character dialog where I would expect them to be and the Help topic doesn't say anything about modifying the shadow effect.
I think the only way would be to do this by hand, setting the main line in the text colour, then a second line in the shadow colour, displaced to the side and below by the required offset.
Apache OpenOffice 4.1.7 on Xubuntu 18.04.3 (mostly 64 bit version) and very infrequently on Win2K/XP