I lost the answer to how to solve my 2 error messages. Please re-send

Here is the first of two error messages I get. It appears on my Dashboard as well as on the page where my posts are listed. Can anyone give me directions on how to fix this one below?

WARNING: NextGEN Gallery Stylesheet NOT Upgrade-safe

nggallery.css is currently stored in /var/www/webfiles/nkscouting.com/web/wp-content/themes/cat4-theme/nggallery.css, which isn’t upgrade-safe. Please move the stylesheet to /var/www/webfiles/nkscouting.com/web/wp-content/ngg_styles to ensure that your customizations persist after updates.