Depends on the post style. Personalized flobs and dividers and such aren't allowed because posting those gives you gold which is considered a form of spam, but changing the font, the font's size, and the font's color are okay, along with indenting the text in your post. (like the way I post, with [indent] [/indent])

Yeah. For the header of a new thread or something (such as your quest thread, exchange thread, etc) it's fine, but you can't use it every post you make.