A cover letter is a letter intended to talk directly to an employer and a chance to sell yourself.
A cover letter allows the applicant to elaborate further on what you can offer the role that you are applying for.
It is vital that you tell the employer what role your interested in, explain why? They you should talk about why you think your appropriate for the vacancy and to demonstrate your passion and commitment to that job and the field in general.
Then you should sell yourself in terms of what skills and abilities you could offer the company, and how past experiences has aided your knowledge to advance.
I believe you should then add a statement out of courtesy to thank the employer, after all they have many CVs and cover letters to read, so out of politeness i thanked them for their time and consideration.
The design element of this piece is similar to that of my cv, I wanted to bring the same continuity to this piece, they will be sent together the majority of the time, so it gives and all round impression that this is me as a 'Brand'. Again I've kept the layout simple with a nice clean piece of work on each page. There is a lot of information in these documents and i personal think simplicity will engage the employer just as much as something that is over worked.
I did look at various different colour schemes and imagery to gage what is more appropriate, and in my personal opinion the most successful Cover letter is on the white background.