fixed some minor dark-theme errors

[jitendravjh]

fixed some minor dark-theme errors. Closes #600

[shravanngoswamii]

@penelopeysm Did we remove the pull_request_target event from Preview workflow in this repo? If we did this for security reasons then, Is there any reason we are still using pull_request_target in remove_preview workflow? I suppose we can use pull_request event in remove_preview too, as we are already not generating docs for PRs from forks! And we should skip running both workflows in case of PR from forks, I guess to avoid unnecessary failures!

Do you want me to update it?

[jitendravjh]

These lines in dark mode are looking a bit weird, but I guess you guys are already changing theme in TuringLang/turinglang.github.io#119 so It's better to fix it there!

Light Mode	Dark Mode

[shravanngoswamii]

Thanks @jitendravjh! I am approving this, code blocks are appearing fine in dark mode too after this PR!