There is not a template for a legal brief in Word, as of 2015; however, there are Word templates that have been created by various third parties that can be used. Law school web sites are an excellent source for legal brief templates that are Word compatible.
Many templates are free, but there are paid templates as well. There are templates for both PC and Mac users, as well as for various Word versions. Because different states have different formatting requirements for legal briefs, it is important to make sure that any third party template follows the guidelines a particular state is bound by.