Styles can be created by accessing the 'Style Builder' in Expression Web.
Click 'New Style' which is located in the Manage Styles Panel. (see screenshot below)
The
Manage Styles Panel is located in the
Left Panel, usually the bottom half. To locate this panel quickly,
click on Panels--> choose Manage Styles Panel.
This will open the Panel.
As a result, the New Style dialog window will open. This is what I refer to as the Style Builder. It's just easier to say! (see the image below.)
The Style Builder is show below

Now that we have the style builder open, let's start creating our Style.
Let's assume we are styling the header of our web page. We already have the div on the page. Now we need to create the CSS Style and apply Properties and Values.
The Header: The best type of Selector to use for the header, in this situation, is an ID Selector. Remember, an ID Selector must be unique. In other words, a particular id selector may be applied to one element on the page, and that's it.
An ID Selector also begins with the # pound or hash sign.
The actual word that we use for an ID Selector is any word we want to use. In this example, we will just use the word "header". Therefore, our Selector for our Style we be #header
#header {
}
Next, we need to set the property value pairs that will control the appearance of the header. for instance, let's specify a height for header.
#header {
height: 80px;
}
We would continue to add property: value pairs until we achieve the look we are shooting for.