You can still apply to ICS if your undergraduate degree is not in the same area as the graduate degree in which you are interested. However, it is helpful if you have taken courses in computer science and math, and/or have some related work experience.
The statement of purpose is required while the personal history statement is optional but highly recommended.
The "statement of purpose" should focus on your academic/research background and interests. The "personal history statement" essay should focus on your personal background and any challenges or obstacles you may have encountered in your academic journey and how you overcame them.