Preview styling improvements (tables, HRs etc)

[dwertheimer]

No description provided.

[jgclark]

Thanks, David. My only comment is to suggest you move the new hexToRgb() function to live in helpers/colors.js, so that can be re-used more easily.
(Come to think of it, I think you've used some sort of chroma library in the past. Does that not have this sort of function already?)

[dwertheimer]

Good suggestions. Moved to colors and used chroma.

[dwertheimer]

@jgclark in Discord concluded that:
"I'm not keen on having table/th/tr/td in ThemeToCSS"
a) the CSS classes should probably be scoped to be Preview-specific
b) Or, potentially, a slightly more general "EditorLayout" class, as that's what you're trying to mimic, I think.

@jgclark You may want to scrap this PR and the code changes and start fresh depending on how you want to implement

I used to think that Preview was a short-term need, but after Eduard rolled back the Mermaid attempt in published pages, I think for me, Preview is going to be necessary for the indefinite future.

[dwertheimer]

@jgclark Nevermind this PR. I am going to start a new one