XF 2.1 How to code pages with the Xenforo code

[Booff]

Is there a way to edit the designs within the pages with HTML/CSS code or do I have to use the Xenforo designs

 

[Overscan]

You can install styles that other people have designed, and you can customise whichever theme you use as a starting point. You can add custom CSS to alter the appearance you your liking. You can write your own complete style if you are so inclined, though that's quite complicated. There are many free styles and paid ones. I used the default theme and customised it to my liking, using a number of custom CSS tweaks from this forum.