As always - if one applies oneself to the act of reading tutorials, one starts to realize what a moron one can be if insufficient study has taken place. To my disgust I am one of the ones.....
The whole 'picture tag nested around a srcset' and taking advantage of several useful attributes - largely provides the answer that I was searching for at the heart of my OP. So the RSD themes DO HAVE mobile first potential o'plenty if one avails ones self of the picture and srcset tags/attributes, etc.
I'll bet there are many useful tutorials,etc., but here is what I found that helped me move off dead center moronville - wrt picture tag and srcset.
https://www.smashingmagazine.com/2014/0 … re-srcset/
After some reading and application, it looks to me like applying picture and srcset to the site images goes most of the way to being responsive, in terms of serving an appropriate image to the screen in use, at least on an image heavy mobile first site.
wavetrainSUP
quietwater paddles
supCAT boards