tyz-tref
Hello
What you are asking is not possible in one single step ( adding lines of codes to the custom.css)
Look at images below explaining what i mean
2 pieces of blocks are using 2 different ol classes
tl-wrap pull-left for the one on the left side
tl-wrap pull-right for the block on the right side
So you should create for the needed size ( <= 320 screen size ) a new css rule moving blocks all to left and using only 1 ol class tl-wrap pull-left
I'm afraid to say that this will be a custom dedicated work which is out of this forum support scope , however my explanation should guide you to the right direction to obtain what you need.
Hope it helps
