Set background-color on search input

[pete-murphy]

Fixes #1408

Description of the change

Explicitly setting background-color: white in the search <input>'s styles, to override UA stylesheet default of background-color: Field which has poor contrast with the input's color: hsl(221.25, 21.62%, 14.51%) in dark mode.

Before

After

Checklist:

• Added the change to the "Unreleased" section of the changelog
• Added some example of the new feature to the README
• Added a test for the contribution (if applicable)

P.S.: the above checks are not compulsory to get a change merged, so you may skip them. However, taking care of them will result in less work for the maintainers and will be much appreciated 😊

[pete-murphy]

I didn't check until after making this change, but Pursuit has different dark mode styles for its search input:

.top-banner__form input {
  border: 1px solid #1d222d;
  border-radius: 3px;
  background-color: #ffffff;
  color: #1d222d;
  font-weight: 300;
  line-height: 2;
  padding: 0.21em 0.512em;
  width: 100%;
}
@media (prefers-color-scheme: dark) {
  .top-banner__form input {
    border-color: #1d222d;
    background-color: #141417;
    color: #dedede;
  }
}

Maybe we should match that? Would mean moving the style declaration to insertStyle in App.purs so we can use the media query. Nevermind, should be even simpler than that, we should be able to just re-use the same styles from Pursuit CSS (and delete a lot of inline styles here) by using the same classes as Pursuit.