While all rust documentation has some pretty nice themes, it's a bother to have to select the proper theme for evey single repo, and sometimes this choice does not affect other modules.
Simply add this to your ENV.
# .zshrc
export RUSTDOCFLAGS="--default-theme=ayu"
There's a better way to do this, it's to modify the Cargo doc. So that's a toml file that is held in the user directory.