Only by writing this way will you shake the editor format stupor evoked by reading all those staid, letter-perfect proposals. Proposals that go nowhere except into the rejection pile. From the very first words of this personal letter, the editor will sense you're different from all the other hacks vying for her attention. Design the letter to be visually pleasing. No, don't rush out and buy a desktop publishing program with fancy fonts, graphics and logs with smiling faces.
Nothing shouts amateur as loudly.
Instead, use a businesslike typeface. Like any letter expressing affection, it should connect with her on a visceral level, acknowledging that she is a person, not only a ticket to publication. Beginning with the salutation, make her feel chosen. You've thoughtfully selected her, whether she's a senior editor or the lifestyle editor or whomever. Address her by name and spell it correctly.