In which order do CSS stylesheets override?

The rules for CSS rule cascading are complex -- rather than trying to paraphrase them badly, I'll simply refer you to the spec:

http://www.w3.org/TR/2011/REC-CSS2-20110607/cascade.html#cascade

In short: more specific rules override more general ones. Specificity is defined based on how many IDs, classes, and element names are involved, as well as whether the !important declaration was used. When multiple rules of the same "specificity level" exist, whichever one appears last wins.

The most specific style is applied:

div#foo {
  color: blue; /* This one is applied to <div id="foo"></div> */
}

div {
  color: red;
}

If all of the selectors have the same specificity, then the most recent decleration is used:

div {
  color: red;
}

div {
  color: blue; /* This one is applied to <div id="foo"></div> */
}

In your case, body:not([input="button"]) is more specific so its styles are used.

Order does matter. The last declared value of multiple occurrence will be taken. Please see the same one I worked out: http://jsfiddle.net/Wtk67/

<div class="test">Hello World!!</div>


<style>
    .test{
        color:blue;
    }

    .test{
        color:red;
    }
</style>

If you interchange the order of .test{}, then you can see the HTML takes value of the last one declared in CSS