Auto merge of #106487 - GuillaumeGomez:fix-kbd-var, r=notriddle

Fix --kbd-color variable name in rustdoc.css

Interestingly enough, it only impacted the dark theme.

Before:

![Screenshot from 2023-01-05 11-03-17](https://user-images.githubusercontent.com/3050060/210754145-c3bb0f50-d35f-4543-bf73-010a4524a803.png)

After:

![Screenshot from 2023-01-05 11-03-05](https://user-images.githubusercontent.com/3050060/210754190-85c2f146-a774-4463-9cd3-9495b7c91bd7.png)

r? `@notriddle`

Author: bors