Anything can be customized, by adding a custom version of the file in your vhost by mimicing the exact file structure of the content type ([vendor]/[bundle]/Resources/views/[file]).
The site is assigned to a specific theme, which in this case is the Custom theme (which is a blank slate that allows easy customization). By default every page is assigned to a basic shell design (shell.html.twig) which determines the look/feel of the theme. But you can edit any given page (from the CMS Admin > Content > Edit > Settings) and select an alternative shell that is available for this theme (You can customize existing shells or create new shells for a theme as well).
If you are assigned to the Custom theme, you can create a customize version of the shell.html.twig inside the TemplateCustomBundle, which is located in Sitetheory/TemplateCustomBundle/Resources/views/shell/shell.html.twig. This extends Sitetheory’s base shell template [Sitetheory/TemplateBundle/Resources/views/shell.html.twig](https://github.com/gutensite/Sitetheory/blob/1.0/src/Sitetheory/TemplateBundle/Resources/views/shell.html.twig) which already establishes the main structure of a template. This core shell template in turn that extends the HTML base template [Sitetheory/CoreBundle/Resources/views/base.html.twig](https://github.com/gutensite/Sitetheory/blob/1.0/src/Sitetheory/CoreBundle/Resources/views/base.html.twig). Reviewing the parent templates will show you which Twig blocks can be extended.
Every page is a “Content” which is assigned to a Content Type, e.g. Article, Profile, etc. To customize the look of all Articles, just add a file called /Sitetheory/ArticleBundle/Resources/views/Article.html.twig. To customize one specific Article, get the unique content ID of that article (from the admin url ?id=xxxx, or in the dev toolbar) and add a file called /Sitetheory/ArticleBundle/Resources/views/Article[ID].html.twig where “[ID]” is the content ID.