If you browse the site in dark mode, then the green header and the black/grey background are the colors of the default site theme. However, when you enter the page of any game, it will switch to the theme of that game, as long as if you have that option enabled in the settings, and the game moderators made a custom theme for it.

The game "NEW SCUFFED EPIC BHOP SIMULATOR 2023 (POG CHAMP)", has a custom theme with black background and red header. That's why you see it that way when you enter the game leaderboard.

k thx