Although you can style every part of a Microsoft Word document individually with unique fonts and colors, it is not a convenient approach since you have to repeat the steps many times, consuming more time and increasing the chances of mistakes, and this is when styles and themes can come in handy.
In the Office apps, styles are collections of fonts, text sizes, and colors that you can apply to different parts of the document (titles, subheadings, quotation, etc.). On the other hand, themes are collections of styles that you can use to format the entire document quickly.
Full story from the WindowsCentral blog...