Use :hover to modify the css of another class?

It's not possible in CSS at the moment, unless you want to select a child or sibling element (trivial and described in other answers here).

For all other cases you'll need JavaScript. jQuery and frameworks like Angular can tackle this problem with relative ease.

[Edit]

With the new CSS (4) selector :has() guide from CSS4.Rocks (archived by Wayback Machine) and :has() guide from MDN Web Docs, you'll be able to target parent elements/classes, making a CSS-Only solution viable in the near future!

There are two approaches you can take, to have a hovered element affect (E) another element (F):

1. F is a child-element of E, or
2. F is a later-sibling (or sibling's descendant) element of E (in that E appears in the mark-up/DOM before F):

To illustrate the first of these options (F as a descendant/child of E):

.item:hover .wrapper {
    color: #fff;
    background-color: #000;
}​

To demonstrate the second option, F being a sibling element of E:

.item:hover ~ .wrapper {
    color: #fff;
    background-color: #000;
}​

In this example, if .wrapper was an immediate sibling of .item (with no other elements between the two) you could also use .item:hover + .wrapper.

JS Fiddle demonstration.

References:

• CSS 2.1 selectors, at the W3.org.

You can do it by making the following CSS. you can put here the css you need to affect child class in case of hover on the root

.root:hover    .child {
   
}

Provided .wrapper is inside .item, and provided you're either not in IE 6 or .item is an a tag, the CSS you have should work just fine. Do you have evidence to suggest it isn't?

EDIT:

CSS alone can't affect something not contained within it. To make this happen, format your menu like so:

<ul class="menu">
    <li class="menuitem">
        <a href="destination">menu text</a>
        <ul class="menu">
            <li class="menuitem">
                <a href="destination">part of pull-out menu</a>
... etc ...

and your CSS like this:

.menu .menu {
    display: none;
}

.menu .menuitem:hover .menu {
    display: block;
    float: left;
    // likely need to set top & left
}