This can be different depending on the specific theme that you are using. For instance, these themes automatically include a centered menubar:http://allprowebtoolsthemes.com/-designerhttp://allprowebtoolsthemes.com/-gatewayhttp://allprowebtoolsthemes.com/-journalisthttp://allprowebtoolsthemes.com/-lifestylehttp://allprowebtoolsthemes.com/-long-tail
All themes are just HTML/CSS but each can be laid out in an entirely different way to accomplish the desired look. Your question does not apply specifically to AllProWebTools, but instead to the nature of HTML/CSS.
You may want to choose a theme from the above list that has the centered menubar that you are looking for, or you may want to view the source HTML and CSS files from these examples to see if you can implement those elements into your current theme.
I hope this gives you a direction to pursue, we are hear to help if you have additional questions!