Just in case you are not familiar with how to do this (taken from documentation)

Custom css is new css code you add to your site
it's either - new css code or css code to override an existing template styling

The file it needs to be added to is called custom_rename_to_custom.css or if renamed custom.css (for the file to work it needs to be called custom.css) - this file is protected from being overwritten by template updates

The path to the file is
\templates\template name\css\custom_rename_to_custom.css

via the joomla backend the path is

extensions tab > template manager > Templates (top left in Joomla 3) > "template name" details and files > Edit css/custom_rename_to_custom.css