Hello @madpugger,
Both #cms and .cms-id-{id} are present on the <body> tag :
<body id="cms" class="lang-en country-fr currency-eur layout-full-width page-cms tax-display-enabled cms-id-4">
So if you wan to target a specific CMS page you have to use .cms-id-{id} (replace "{id}" by the id of the CMS page).
If you want to target all CMS pages just use #cms in you CSS file 😉