This small feature is available for the new XAF Web UI only, which is enabled by default in new projects starting from v15.2.7 and on. In short, the CustomCSSClassName attribute allows you to set a CSS class for a layout item or a group. Let's see how this works in practice.
For instance, to customize the color and size of the FullName view item, add the FullNameCSS CSS class in YourSolutionName.Web/Default.aspx file exactly as you would do this in a regular non-XAF ASP.NET application:
<!--...-->
<head runat="server">
<title>Main Page</title>
<meta http-equiv="Expires" content="0" />
<style type="text/css">
.FullNameCSS
{
font-size: 25px;
color: darkblue;
}
<!--...-->
The result is demonstrated in the image below:
Please let us know whether this new customization option is helpful for your projects.
BTW, if you are getting started with the new Web application style, you may find this article helpful: New Web UI FAQ