So are you trying to set "default" STB file (for use with R14 and previous drawings - as your photo shows), or are you trying to set the current STB file for a particular layout?
IF the former...
IF the latter.....
(setq o (vla-get-Output (vla-get-Preferences (vlax-get-acad-object))))
(vla-put-PlotPolicy o 0)
(vla-put-DefaultPlotStyleTable o "monochrome.stb")
See this post for an example of setting the "stylesheet" for multiple layouts in multiple drawings.
If you just want to operate on the current layout, then here is a lisp example: