When I worked at RNIB, I changed my default desktop colour scheme because the default windows scheme gave me headaches and eye strain. The department of unintended consequences then stepped in and handed me a neat way to know whether a site was missing it's body background colour without having to check any code, because it changed the default browser background from white to muddy grey. Since moving to using a mac, I've grown used to the white default background in the browser, and this morning, had a moment of panic when I realised that I wasn't sure if I'd specified the background colour in the templates I'd built (and which were delivered to the client) last week.
I had, of course, but I realised in that moment that it'd have been much easier if I went into the settings and changed the background colour away from white (to grey, but you could choose whatever makes you happy), just to avoid that kind of panic in future. So that's what I did.
Disappointingly, the first site to fall at that particular hurdle was the Firefox default Google search home page.