you are right, the customerstyles.css is the right file.
font-size: 33px !important;
change the 12px to what are you fine with.
Please make sure to clear the browser cache to see the changes on live on an browser reload (On Google Chrome it is STRG+F5 for example).
Please be aware though that this might change with the release of 13.2: What about my web interface customizations as of PRTG 13.2?
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