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...
Code:
(setq o (vla-get-Output (vla-get-Preferences (vlax-get-acad-object))))
(vla-put-PlotPolicy o 0)
(vla-put-DefaultPlotStyleTable o "monochrome.stb")
IF the latter.....
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:
Code:
(vla-put-StyleSheet
(vla-get-ActiveLayout
(vla-get-ActiveDocument (vlax-get-acad-object))
)
"monochrome.stb"
)