There were two white strips in the thumbnail. This is due to the thumbnail being a bit wider than the recommended size, so the image was fit into the space. In the light theme, this is unnoticeable; however, seeing this in the dark theme makes the thumbnail look like it was poorly cropped.
If the white is from the background of the page, it would probably be a quick change. Sorry if I’m being demanding, but it creates a sense of un-professionality for the game.
In conclusion, It would likely look much better as black stripes.
As you can see in this screenshot the white bars are from the image, not the background of the page. When you remove this image source the background is actually in the dark theme:
As a result, this is as a result of the thumbnail not being the recommended aspect ratio, and you should probably modify it to meet the recommended size requirement.
