WordPress normally gives you default templates or templates that one could buy online. It is pretty standard with respect to appearance. But one could customize the themes and colors with css. And you could also play around with the layout if you know html and php.
Before you can make changes to css file you would need to know where it is located right? In the WordPress admin panel, goto Appearances and then click Editor. It would give you a list of files on the right. The css file would be right at the bottom.
Regarding layout you could work with the php files. WordPress is developed using php. So, most of the files are php templates where one could make changes. There is a file called header.php, which would allow you to make changes to the header of a page. And once a change is made on the header, it would get reflected on the other pages as well.
People would definitely need to know php in order to make changes to those files. There are other files like footer.php and page.php which would allow you to modify the footer and the page respectively.