How to exclude particular class name in CSS selector?

I'm trying to apply background-color when a user mouse hover the element whose class name is "reMode_hover".

To clarify, my goal is to color the first element on hover but not the second element.

<a href="" title="Design" class="reMode_design  reMode_hover">
    <span>Design</span>
</a>

<a href="" title="Design" 
 class="reMode_design  reMode_hover reMode_selected">
    <span>Design</span>
</a>

/* do not apply background-color so leave this empty */
.reMode_selected .reMode_hover:hover 
{
}

.reMode_hover:hover 
{
   background-color: #f0ac00;
}

One way is to use the multiple class selector (no space as that is the descendant selector):

.reMode_hover:not(.reMode_selected):hover 
{
   background-color: #f0ac00;
}

<a href="" title="Design" class="reMode_design  reMode_hover">
    <span>Design</span>
</a>

<a href="" title="Design" 
 class="reMode_design  reMode_hover reMode_selected">
    <span>Design</span>
</a>

In modern browsers you can do:

.reMode_hover:not(.reMode_selected):hover{}

Consult http://caniuse.com/css-sel3 for compatibility information.

Method 1

The problem with your code is that you are selecting the .remode_hover that is a descendant of .remode_selected. So the first part of getting your code to work correctly is by removing that space

.reMode_selected.reMode_hover:hover

Then, in order to get the style to not work, you have to override the style set by the :hover. In other words, you need to counter the background-color property. So the final code will be

.reMode_selected.reMode_hover:hover {
  background-color:inherit;
}
.reMode_hover:hover {
  background-color: #f0ac00;
}

Fiddle

Method 2

An alternative method would be to use :not(), as stated by others. This will return any element that doesn't have the class or property stated inside the parenthesis. In this case, you would put .remode_selected in there. This will target all elements that don't have a class of .remode_selected

Fiddle

However, I would not recommend this method, because of the fact that it was introduced in CSS3, so browser support is not ideal.

Method 3

A third method would be to use jQuery. You can target the .not() selector, which would be similar to using :not() in CSS, but with much better browser support

Fiddle