For example, you can remove the title of a form by adding "&ttl=0" to the URL, you can change the font by adding "&f=FONT_NAME", the text color by appending "&tc=HEX_VALUE" and the background color using "&bc=HEX_VALUE".
Note that this works only for the embedded forms, which have a special URL. Here's the address of the customized form displayed below:
http://spreadsheets.google.com/embeddedform?key=rwP1POieyAS3nnI7OqD7sHQ&ttl=0&f=Verdana&tc=0022ff&bc=f9fff9
Another option is to copy the HTML code generated by Google and to style it using CSS.
{ Thanks, ahab. }