I'm glad to hear there's a proper solution planned at some point since mine is somewhat hacky, but I'm not surprised there's no clear timeline.
I've been wanting to set LessWrong as my home page for a while but kept avoiding it because the site is so visibly bright.
I looked up ways of viewing the site without hurting my eyes and found https://www.greaterwrong.com, but didn't really like it.
Then I found https://github.com/lfaucon/lessbright, which looked right, but wasn't technically the real LessWrong site (which meant it couldn't tell I was logged in, among other things).
But I already use Stylus to apply stylesheet modifications to webpages, so I looked at lessbright's source code, found the CSS code it used to change the site's appearance, and applied that to a new Stylus style (although my second monitor doesn't have very good contrast, so I used 100% instead of 92%):
And now the site looks exactly how I want it!
Just figured I'd share this, in case the site's excessive brightness annoys anyone else.