https://github.com/rust-lang/rust/pull/74361 just got merged however, people expressed concerns so we decided to merge it to fix the current issue and have a first version to debate upon.
An alternative suggested was to use an SVG image and then use CSS to change its color depending on the selected theme.
Another one was to force crates owners to provide multiple version of their logo for the different themes.
The last one was to change the CSS rule we used to something a bit "simpler".
I think that we should allow crate owners to provide light and dark versions of a logo, and in the absence of both, draw the provided logo with a light background even in a dark theme. That's the only way that we can maintain the integrity of people's logo designs. Given that the majority of crates don't provide a logo, having light and dark versions of the Rust logo will cover most cases.
This would be fine as long as the logo and the light background added used no more than say 10% of the default area of display on a typical desktop/laptop screen. There is nothing worse than having everything dark and using your computer in a dim or dark area and then switching to an application or web page where suddenly most of the screen is bright/light. It feels like someone just shot a laser beam into your eyes (which considering that most screens are now OLED, isn't far from the truth).