You may like to know that movable type has quite a few nice responsive themes.
While they are quite popular on the Japanese market, they are still to be unveiled on the English-speaking one.
Let me take initiative and introduce them to you right away.
Included in the Package
Two of these themes are part of the default package and could be seen online at:
Available as Plugable Themes
There are a few others that you could find out more about here:
Among those themes, I chose and installed a few that you could also preview at:
4) Bootstrap 3
There are a few more available in Japanese, but that one could translate in English without much effort (I could also do it). They are:
5) SEO Tuning Eiger (some phrases in Japanese, but they could easily be translated)
6) @Style Bootstrap Theme for Movable Type
What do you think?
I would say that they look quite promising and there is a great potential for adopting any of these themes and eventually customizing them to suit your needs.