You need to save your CSS in a file (e.g. under Zettlr\data\html-custom.css), then add that file in the HTML.yaml by adding a line like
css: [C:\somepath\Zettlr\data\html-custom.css]
To use sans-serif fonts, just add this to the css file:
body {font-family: sans-serif;}
This will overwrite all the default styles, so it will not look good. To make it look nice again, fill out the css file more, e.g. with the template from here. There is also a way to get pandoc's default css, I can paste it here if you're interested.