[larrybradley]

This moves all of the photutils custom.css into the theme, as discussed in astropy/photutils#2530. #62 upstreamed the earlier photutils tweaks, and this adds the later dark-mode work. With this change, photutils (and regions, which mirrors it) can delete their custom.css and render identically.

• Dark mode gets a darker #151515 background, neutral gray text, borders, and backgrounds, and link, muted-text, and inline-code colors checked for contrast against that background.
• Adds brighter code comment, prompt, and number colors, a visible text-selection color, zebra table rows that separate the header from the first row, and updated admonition, danger, and target-highlight colors.
• Adds the photutils heading sizes and spacing, and changes the accent color from #ff5000 to #ff9a6c. To my eye, #ff9a6c fits much better with the steel blue and orange palette, while #ff5000 clashes with it.
• Makes Nord the default dark-mode Pygments style, since the code color overrides are tuned for it.

CC: @Cadair

[Cadair]

Thanks @larrybradley. It might take take me a few weeks to get to this as well are on vacation soon, and I want to figure out what of this should go up to sunpy.