Common rules
Always add <style> at the top to override the styling that Help Centre uses.
<style>
h1 { font-size:24px; }
h2 { font-size:22px; }
h3 { font-size:20px; }
h4 { font-size:18px; }
h5 { font-size:16px; }
table th { font-size:14px !important; text-align:left !important; }
table td { font-size:14px !important; text-align:left !important; }
</style>
!important indicates to Help Centre to override their own style if they have styling set for the same element