Currently, the `site.giscus.theme` option is ignored because
`giscus.liquid` always computes the theme as light/dark based on the
current site theme. This PR allows users to configure separate light and
dark giscus themes in `_config.yml`, which will support dynamic updates
when switching between light, dark, or system themes.
Fixes#3269
Earlier there was a tag in the `_config.yml` file to exclude the posts
from the search results. I could no longer find that option and hence
modified the `search.liquid.js` file.
It basically checks if the `posts_in_search` tag is true or not.
