To programmatically set a theme for a web page, the user needs to know the ASP.NET page life cycle events and how they are used to modify the page's behavior.

Now, let's go through each option and explain why it is right or wrong:

A. Init event: This event occurs when the page is initialized, and it is too early in the page life cycle to set the theme. This event is used to set the initial property values of controls on the page.

B. PreInit event: This event occurs after the page has been initialized but before it has been loaded, making it the ideal place to set the page theme programmatically. This means that option B is the correct answer.

C. Load event: This event occurs after the page has been loaded, which is too late to modify the page's theme. This event is used to perform any final initialization steps.

D. Can be set only at design time: This option is incorrect. While themes can be set at design time, they can also be set programmatically at run time using the PreInit event.

Therefore, the correct answer is:

The Answer is: B. PreInit event