After you configure one or more sites for your Open edX installation, including their domain and platform names, you can set up a theme to use for each site.
To override the files that constitute the default Open edX theme, you create replacements for one or more of those files, place them in file paths that are constructed and named in parallel to the default file locations, configure your Open edX instance to use the files in your theme’s directories instead of the default locations, and then compile the theme.
EdX first looks for files in your theme directories, and uses any file that matches the exact file path and file name of a default UI file. If matching files are not found, then Open edX looks in the default location.
For more information about Open edX sites, see Configuring Open edX Sites.