website: dark color them renders code unreadable

[matthiaskrgr]

Description

https://rust-lang.github.io/rust-clippy/master/index.html#/panic

select the "coal" theme for example

[xFrednet]

cc: @GuillaumeGomez & @Alexendoo

[GuillaumeGomez]

The problem is fixed in #13539. It was simply that the highlighting was not run.