If you want to add custom code to your website you can use Field formatters, Blocks and Custom DX8 elements.
Field data from content entities can be rendered on a DX8 template either as raw data via tokens, or using the field’s default Field Formatter. If using a Field Formatter, the output will be exactly as defined by the field module. This means DX8 will work seamlessly with core fields and contributed modules that supply their own layout and templates for their field output. Developers are also able to create custom fields with custom formatters and DX8 will render these exactly as the developer has defined.
The simplest way to inject developer created layout and styling into the DX8 templates is to define them as a standard Drupal block. With that approach you can create any layout you need, bring in blocks defined in drupal.org modules and retain 100% control of the layout of that block.
Custom DX8 elements
The downside of using a block is there is no way to give a site builder options from within the DX8 sidebar editor. To do this, you can create your own DX8 element and define its form using the Plugin API.