Use html instead of root for scss/css examples

[stacyk]

For instance, https://github.com/oddbird/accoutrement/blob/5b674faa4d68db2413c282267ffd51ce69032ce5/sass/_core.scss#L131 we are using :root as a default place to declare our custom properties. I learned this trick from a super smart person during a virtual conference I attended last month.