"The good news is hell is just the product of a morbid human imagination.
The bad news is, whatever humans can imagine, they can usually create." - Harmony Cobel
you would have to first completely rewrite the code. that style, like almost all 3.0 styles, is done with tables. it is very hard to make a style written with tables to be responsive.
there were a few styles written for phpbb 3.0 that were already responsive.
why not upgrade to phpbb 3.2 and then you can use a similar minimallistic style that will already be responsive.