The help text is currently pretty hard to read. Thinking we should rewrite & reorganize, and split into a couple paragraphs. Worked with @michelleweber to come up with this:

Add your own CSS code here to customize the appearance and layout of your site. Learn more about CSS.

When using a keyboard:

• In the CSS edit field, Tab enters a tab character.
• To move keyboard focus, press Esc+Tab for the next element, or Shift+Tab for the previous element.

The edit field automatically highlights code syntax. You can disable this in your user profile to work in plan text mode.

I'm not sure if "edit field" is the write term; @helen do you have any opinions here?