Enhancement: Change true black in graphics to a lighter shade of black because of black smearing on OLED screens

[puyoxyz]

下記の問題を説明してください。 スクリーンショットと診断情報を含めてください。

Describe the problem you are having below. Please include a screenshot and the diagnostic information.

Because of OLED smearing/black smearing, pixels that are true black (#000000) lag behind on OLED screens. Since notes in taiko-web have a true black border, on OLED screens they look bad since the border looks like a long C shape. (basically, the black border lags behind the rest of the note)

(Screenshots and diagnostic information don't matter in this issue as it's a problem with all OLED screens and a problem with the assets, not the actual code)