Change Layout MIN width if width is lower than X pixels?

Hi!

I have a website with 70% fluid width, minimum 640 pixels and maximum 950 pixels, which looks great on laptops, tablets and big screens.

But, when using an iPhone I wish to lower the minimum width to fit the screen better.

Is it somehow possible to override the theme settings so that minimum width is 640 pixels down to 321 pixels and when the size of the website measures 320 pixels the minimum width changes to 320 pixels?

Edit: Whops, wrote "pixels" four times in one sentance