It is recommended that you add both types of validation to your forms, primarily because client-side validation does not get applied if JavaScript is turned off on the browser.

Client-side validation is based on JavaScript code that is run on the user's browser. The validation is processed when a user submits a form, and will display error messages in a JavaScript alert box. Server-side validation is processed by the hosted server, where the error messages can be displayed as text on the form.

The server-side validation will need to be applied to the page that the form is being submitted to.  For example, if you have a form on index.php that submits to result.php (<Form name=“test” action=“result.php”>), then you will need to apply the server-side validation to the result.php page. 
When the form action attribute is blank (<Form name=“test” action=“”>), the form will post to itself. Such cases require that you apply the server-side validation to that page.

If your index.php page submits to your result.php page, you will need to add the form bindings from the index.php page to the result.php. With Validation Toolkit installed, and the result.php page open, choose Add from the Bindings panel, select Form Data, choose index.php, and click OK.

Notice that the form from index.php is now listed in the Bindings panel, allowing you to select the form when applying the server-side validation. (index.php and results.php are used as examples only and are not required)

Yes, you can apply CSS to your form elements. Instructions on how to accomplish this are provided in the following How to:

Applying CSS styles to form elements with Validation Toolkit (PDF)