If you're like most technical staff, you are invested in your proposals and have thoughts on the look and feel of what you are turning in. So you've probably run up against marketing staff that insist on certain color schemes, fonts, etc. and don't want to make your suggested changes. Why won't they listen?
Just as you went to school to learn your skills, they did, too. They participate in continuing education, and continually hone their skills. "Branding" is one of those skills.
Think of a certain brand that uses very specific shades of red and yellow, as well as 'Golden Arcs'. You don't even have to see the name to know what the company is, and to have feelings associated with them. They have used those colors and logos for years, and used their advertising to train you how to feel about the company and their products. That's what branding does!
Your marketing staff and graphic designers have worked hard to create color schemes, logos, and themes that project the image your company wants to show the world. Through your business development efforts and accomplishments on your projects, you train the clients (and potential clients) how to feel about your company when they see that branding.
The 'look' of proposals and marketing materials is not arbitrary. Each element has been carefully crafted. Embrace the vision of your marketing department!