Respect color scheme override

[ZibingZhang]

Fixes cases not covered by #1720.

Problem

We can see that the background color is overridden by the rule below despite the fact that the chosen color theme is `light`.

@media (prefers-color-scheme: dark) {
  table.import-preview td del {
    background-color: #001919 !important;
  }
}

Solution

How did you solve the problem?

I add the rule below to override this CSS rule which we can see fixes the issue.

html[data-theme="light"] table.import-preview td del {
  background-color: #ffe6e6 !important;
}

Acceptance Criteria

Have you written tests? Have you included screenshots of your changes if applicable?
Did you document your changes?

[coveralls]

coverage: 100.0%. remained the same
when pulling c7de9a0 on ZibingZhang:zizhang-respect-color-scheme-patch
into 1be341e on django-import-export:main.

[matthewhegarty]

Thanks - would you mind adding to changelog:

Fix issue with highlight when using 'light' color scheme (#1728)