In Preferences → Appearance, you need to enable Custom CSS. No need to choose a new theme (though you can if desired).
This allows you to edit a file called obsidian.css at the top level of your vault. (If no such file is there, you may need to create it yourself—just create an empty text file and save it as obsidian.css.) Copy and paste the CSS code above into this file, save the file, and then refresh Obsidian. (cmd+r or quit and relaunch.)