The CONTENTdm Website Configuration Tool offers a large number of configuration options and settings that require no knowledge of HTML, CSS, or JavaScript. However, the Website Configuration Tool also gives you various upload dialogs to add your own HTML, CSS, or JavaScript files to modify the CONTENTdm website UI and create your own advanced UI customizations.
If you are familiar with customizing the 6.x CONTENTdm website, the new responsive CONTENTdm website handles advanced customizations in a very different way. The new responsive UI is built on the React JavaScript framework. There is no PHP or jQuery involved in the new website. This means your customizations need to rely largely on CSS and JavaScript.