My forum is responsive on most devices, but it seems something goes wrong on tablet screen sizes. I was on my iPad the other day, and it looked very similar to this (I was too lazy to get my iPad, so this is a shot of a scaled down window on Safari)
Looks like you have a huge margin set instead of a maximum content width. Which would give the sides at high res and lose them as it goes lower. Stock CSS eventually bumps the margin altogether at really low res. It's possible a percentage was set for width instead causing a similar effect.
I am running out the door so no time to check the css.
You'll want to change your Style Properties -> Page Width Controller's Width, currently the width is set too 65%.
You could remove the width of 65% and just apply a max-width of your desired value. Basically when you set it to 65% it'll use 65% of the screen width all the way down to the smallest resolution which isn't really idea for most devices.