Came to the Parenting site and it took a while to find the search functionality. This blends into the top bar so much it would certainly take forever to find the search window being hidden in plain sight.
I have to agree - it's terribly hard to pick out at a glance, and my eyesight is pretty good. Compare with Stack Overflow, where it has a nice white background like all the other textfields:
It'd be one thing if the rest of the site had a dark theme - there'd be less contrast overall between the top-bar and the rest of the page, and a grey box might jump out a bit more readily... But with the big field of white below, greys and blacks just kinda blur together.
Here's what it'd look like on other sites if it had a white background:
To see this yourself, here's a one-line script to paste into your console:
$(".top-bar .js-search-field").css("background-color", "white")
Could readily do the same with user CSS if desired.