Hard codew the "Your bridges" background colour

When working on #45058 (closed), the design in Figma used a token called background/color/canvas-v2. This is currently #F9F9FB in light themes, #1C1B22 in dark themes, and Canvas in high contrast themes. However, this doesn't match any of the tokens we have in our CSS. Moreover, there wasn't a close one that I could find either.

So I went with --background-color-card, i.e. --background-color-box, which is white in light themes, #23222b (gray 80) in dark themes, and Canvas in high contrast themes.

Screenshot of the bridges settings in a dark theme.

Screenshot of the bridges settings in a light theme.

NOTE: this is basically just the background for the box lists. But the border is lighter than the box list.

@felicia what do you want me to do here:

  1. Keep the background to match the box lists, but with a lighter border (as it is in the screenshots).
  2. Keep the background to match the box lists, and make the border the same as well.
  3. Hard-code #F9F9FB/#1C1B22/Canvas for this one display.
  4. Match some other background token from the settings? E.g. the moz-promo banners, like the "Same settings, new look" banner, use color-mix(in lch, currentColor 8%, transparent) for both light and dark themes.
  5. Hard-code our own color-mix using currentColor or some other token.