 changing the appearance of your site what is a theme  a theme is a set of files that define the visual look and feel of your site the core software and modules that run on your site determine which content including html text and other data stored in the database uploaded images and any other asset files is displayed on the pages of your site the theme determines the html markup and css styling that wraps the content several basic themes are supplied with the core software additional contributed themes can be downloaded separately from the download extend page on drupalorg  or you can create your own theme what is a base theme  a base theme is a theme that is not meant to be used directly on a site but instead acts as a scaffolding for building other themes the core stable 9 theme is one example other base themes can be downloaded from the download extend page on drupalorg  what is a layout  a layout is a template that defines where blocks and other pieces of content should be displayed the core layout discovery module allows modules and themes to register layouts and the core layout builder module provides a visual interface for placing fields and blocks in layouts for entity subtypes and individual entity items see managing content structure for more on entities and fields  changing site appearance overview the main way to change the overall appearance of your site is to switch the default theme the core layout builder and layout discovery modules allow you to define layouts for your site s content and the core breakpoint module helps themes change appearance for differentsized devices see the related topics listed below for specific tasks additional resources extending and customizing your site drupal user guide  theming drupal 