Page layouts can be found in Site Manager>Development>Page layouts. The master page may not have a shared layout so you would have to look at the template then in Site Manager>Development Page templates and modify the layout there.
Depending on the developer, the CSS could be stored in multiple different places. Kentico best practice is to use the CSS Module which can be found in Site Manager>Development>CSS Stylesheets. You might also find it in the file system under /App_Themes/