[cyberbeing]

There is one thing in the changelog which seems to be the reason for the red backgrounds:

情報が更新されると、ダウンロードリストや履歴が赤く表示されます。

Which translates to: When information is updated, the download list and the history are displayed in red.

While not very descriptive, one of the things which it seems to be related to is when the rating statistics and reviews are updated to show how many good/bad/undefined ratings there are. By right clicking and then clicking on View, it will show you some information about the download in the history section and the red background will then disappear for that download. This makes sense because you have then viewed the updated information and it's no longer new.

When the search tree turns red it's because you've gone over 65536 search results and it will no longer add anymore results until you delete enough search terms to bring the results below 65536.