Yes. you can do this.
1.Right-click in the embedded Crystal Reports Designer and select Set CSS Class.
2.In the Object Scope field, select the objects or sections you would like the class to be applied to.
3.Enter the name of the class in the CSS Class Name field.
Tip Use CssClass in the Properties window to view, edit, or delete the class values for a specific report object or section.
4.Click OK.
5.Save the report.
6.Access the .aspx file in your project.
7.In the HTML tab of the .aspx file, reference an external style sheet that has the class already defined, or enter the class value.
For example:
<HTML>
<STYLE>
.classname
{
background-color:808080; border-color:000000; color:000000; font-family:Arial; font-weight:bold; font-style:italic; }
<STYLE>
</HTML>
Source: MSDN
I Hope it will help you.