Recoll and the underlying search Xapian are incredibly powerful. I am surprised that almost all server side search use such bloated frameworks. Xapian seemed really easy to theme but their site and docs need some love. I also don't recall, but if they had a way to parse JSON search hints I'm sure that it would be perfect as a lightweight server side search for static sites. Not that JS search isn't cool, but until browsers implement a native site searcb feature, it is nice to have a way to search from non-JS browsers and clients.