[html] How to set a fixed width column with CSS flexbox

CodePen: http://codepen.io/anon/pen/RPNpaP.

I want the red box to be only 25 em wide when it's in the side-by-side view - I'm trying to achieve this by setting the CSS inside this media query:

@media all and (min-width: 811px) {...}

to:

.flexbox .red {
  width: 25em;
}

But when I do that, this happens:

http://i.imgur.com/niFBrwt.png

Any idea what I'm doing wrong?

In case anyone wants to have a responsive flexbox with percentages (%) it is much easier for media queries.

flex-basis: 25%;

This will be a lot smoother when testing.

// VARIABLES
$screen-xs:                                         480px;
$screen-sm:                                         768px;
$screen-md:                                         992px;
$screen-lg:                                         1200px;
$screen-xl:                                         1400px;
$screen-xxl:                                        1600px;

// QUERIES
@media screen (max-width: $screen-lg) {
    flex-basis: 25%;
}

@media screen (max-width: $screen-md) {
    flex-basis: 33.33%;
}