There’s no substitute for clear, compelling content in your sales proposals. Easy to understand language, diagrams, images, and/or video that makes it clear you understand the prospect’s problem(s), and have the best path to the solution. A lot of people waste too much time on worrying about font colors and other graphical niceties that just don’t matter in most cases.
That said, a little visual appeal to help your proposals stand out doesn’t exactly hurt.