I found a way to run jsampler classic with gtk theme, just by adding necessary -D option to java command, found in /usr/bin/jsampler.
I like it because most pro-audio or multimedia oriented distros already have nice dark themes (qtcurve allowes to get very nice look).
However, mentioned mode switcher still uses default blue metal skin.
There are currently no plans on our side to implement your suggested feature in near future. However you are always invited to provide patches implementing the features suggested by you.
I'm going to close this report.