1. Navigate to Site explorer > Forms and edit the form you'd like to make changes on.
2. Add a new field to the form and then click on the small "pencil" icon when you hover over the field in the right-hand pane. This will allow you to edit the field's properties.
3. Look for a label called "Custom CSS class".
4. For illustration purposes, we will add a new class called "testEntry".
5. We'll add the following to a CSS stylesheet (under Site explorer > Style sheets):

.testEntry {
    color: red;

Notice the "." character in front of "testEntry". In CSS code, this means that you're referencing a CSS class.
The two properties listed above will give the label next to the field a color of "red" and make it "bold".

6. Save the stylesheet and go back to the view mode of the form you're working with. You may need to "hard" refresh the page to see the changes (Crtl + F5 on a PC).

Note: CSS class names must begin with an underscore (_), a dash (-), or a letter(a–z), followed by any number of dashes, underscores, letters, or numbers. You cannot start it with a number.