Dark/light mode toggle now available in core

[Kevin7 (Kévin .)]

Here’s an example: when I switch Discourse to Dark Mode, some button texts remain black. I believe it would make more sense for the text to be white. However, I found a solution using CSS that I think works well to fix this issue.

$button-text-color: dark-light-choose(#ffffff, $primary);

:root {
  --button-text-color: #{$button-text-color};
}

.btn-primary {
  color: var(--button-text-color) !important;
  
  .d-icon {
    color: var(--button-text-color) !important;
  }
}