Removing opacity on featured image; CSS snippet breaks my site

I’d like to remove the partial opacity on the featured image on my homepage. I tried the CSS snippet that was provided on the CSS snippets page, but after I did, my homepage just gave an error message. I’m not CSS-savvy enough to figure out what it means. This is the error message that shows on the page after I paste the CSS snippet:

Fatal error: Uncaught Error: Call to undefined function ct_tracks_sanitize_css() in /home/quarkadmin/public_html/wp-content/themes/tracks/functions.php:598 Stack trace: #0 /home/quarkadmin/public_html/wp-includes/class-wp-hook.php(286): ct_tracks_custom_css_output(”) #1 /home/quarkadmin/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(true, Array) #2 /home/quarkadmin/public_html/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #3 /home/quarkadmin/public_html/wp-includes/script-loader.php(1447): do_action(‘wp_enqueue_scri…’) #4 /home/quarkadmin/public_html/wp-includes/class-wp-hook.php(286): wp_enqueue_scripts(”) #5 /home/quarkadmin/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array) #6 /home/quarkadmin/public_html/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #7 /home/quarkadmin/public_html/wp-includes/general-template.php(2614): do_action(‘wp_head’) #8 /home/quarkadmin/public_html/wp-content/themes/tracks/header.php(8): wp_head() #9 /home/quarkadmin in /home/quarkadmin/public_html/wp-content/themes/tracks/functions.php on line 598

Any idea what’s going on?