Hi there,
in the HTML block you can add your CSS, simply wrap it in style tags ie.
<style>
your CSS here
</style>
as a general rule, if you’re adding a few lines of CSS, then don’t be bothered doing this, it won’t make any real performance improvements over adding it to the customizer.
IF however your’re loading 100 to 1000s of lines of CSS on a specific page then you will want to consider a child theme and enqueueing stylesheets for that.