Overriding theme defaults
If you use the theme like a Ruby gem, to locate theme’s files on your computer
use bundle info --path simple-purple-theme command.
Jekyll will look first to your site’s content before looking to the theme’s defaults for any requested file in the following folders:
/assets/_layouts/_includes/_sass
Layouts and includes
To replace layouts or includes in your theme, make a copy in your _layouts or
_includes directory of the specific file you wish to modify and change it.
Assets
To replace any file from theme’s assets, make a copy in your assets
directory of the specific file you wish to modify and change it.
Stylesheets
Note: To add or modify any stylesheet you must copy the main sass file,
_sass/styles.scss into the _sass directory in your site’s source.
To replace any stylesheet in your theme, make a copy in your _sass directory
of the specific file you wish to modify and change it.
To add your own styles copy the file user.scss in your _sass directory and
change it.
more information
See Jekyll’s Overriding theme defaults page for more information.