rustdoc: theme-dependent logo system

[tesuji]

The current logo in Ayu theme is difficult to see:

As noted by @Cldfire in #71237 (comment):

Orange could potentially work, yes, and I agree that the logo will need improvement. However, I would say that this discussion is out of place for this PR; some sort of theme-dependent logo system needs to be added to rustdoc which needs to be discussed and implemented elsewhere, imo.

This issue is served as a tracking issue for that.

@rustbot modify labels: T-rustdoc

[Cldfire]

Thanks for making this issue! 😄

[GuillaumeGomez]

The best solution would instead to provide a way to change the image border and/or background, which would be much simpler than having a different image for each theme.