CSSの色の選択が上書きされるのはなぜですか？

[jahan_gagan]

Use your browser’s DevTools to inspect the main content area and identify which CSS variable controls the background color.
Then, override it in your theme’s CSS like this:

:root {
  --primary-low: #F6F6F5;
  --primary-very-low: #F6F6F5;
  --d-topic-list-background-color: #F6F6F5;
}