its called CSS, cascading style sheets
a basic tutorial is
http://www.w3schools.com/css/
simply you put a link in the head of each file to the stylesheet
and in the stylesheet define the look of the page
every page looks like the style
change the style sheet, change every page
When you get some code written, it is very much the same as the style in <head> or the width= background= attributes of plain vanilla html, there is a forum dedicated to css, next door to this one,
html & CSS forum and there are code mavens who can answer and suggest
CSS :: no longer need tables for layout, position every element where you want it, tighter faster code, less to debug