Serendipity includes form elements within posts (comment form) and within sidebars (quicksearch, template chooser). As theme designers we should be aware of these and style our themes appropriately.
Form elements are input boxes, select drop downs, textareas, checkboxes, radio buttons, submit buttons etc. All of these elements are styled automatically by your browser, but can take on properties from their parent container. For this reason you owe it to your users to test them, and if necessary, to style them.
First, take a look at the quicksearch input box. Is it the right width, and if you start typing inside it, is the font style, size and color as you intended?
Second, take a look at the template chooser. You'll notice that one of the themes has a very long name. Does this throw out the design of your theme, or have you anticipated this.
Third, take a look at the comment form below this text. Are you happy with the width of the elelments, do they all sit in the position assigned to them, or do some of them seem unnaturally large?