I have checked your site. The file which is containing the related css is style.css inside your
wp-content/themes/pinboard/ folder. However you should not edit that file directly, because that way if you update the theme or WordPress, your settings will be lost. I suggest you to create a child theme, and inside the child theme's folder create a style.css file with the proper contents, and paste your css modifications there. I have made a detailed tutorial video on how to create a child theme, which you can check out here [ Moderator note: Please use Codex links when possible and not send send to your own site. ]
If you need any help, just drop a line here.