Broken code blocks because of syntax highlighting

[bartaz]

Code blocks on charm pages have broken look with some weird white background and additional padding:

https://charmhub.io/openldap-charmers-openldap - but it happens on any others as well

It seems there is some syntax colouring theme added that messes up Vanilla styling:

Vanilla has a built in theme for Prism library.
If you want to provide highlighting for other library please follow the theme from Vanilla and only change code token colours, not background: https://github.com/canonical/vanilla-framework/blob/main/scss/_base_syntax-highlighting.scss