As has been much commented on since the re-vamp of the forum, the colours and bright background can be a bit much for some of us. I had tried a few different web-browser extensions in the past to try to overcome this for this site and others that I struggle with, and I now seem to have found a couple which are both reliable, easily tweakable, and totally free.
I'm not sure if It's OK to link these here, but they are both non-commercial open-source software, and free unless you choose to donate. I have no connection to the producers other than as a user, and I really do find them very useful, as I think many others with light sensitivities might do; so hopefully the mods will deem it OK.
Unlike many others, it doesn't just invert the colours for a dark background, which often looks really awful, and doesn't work if the background is already dark! It has a very clever algorithm that keeps as much of the existing colour theme as it can, but tries to make the background dark, and everything else show up against the background. You can even turn it on and off, and control the brightness, colour saturation, etc. automatically for individual websites (great for toning down that hideously livid purple!) As I say, it's totally free, and is available for Firefox, Chrome, and Safari. I've been using it now for a few weeks, and it seems very reliable. It's only fair to say that there is a down-side; websites will load a bit slower while it does its magic on the colours, more so when the page is very complex, and it doesn't always work well for images that have a transparent background. But on the whole, it's made my browsing experience far more comfortable!
This one looks at your computer's clock, and slowly changes the screen colour so that it becomes less blue around sunset, and back again at sunrise - so that the light from your screen is a better match for how natural daylight changes over the day. This should help avoid eye-strain, and may help with the problem of blue light from screens messing with sleep patterns. Unlike the other one, this is an application that works for the whole screen with all applications, though you can exclude particular programs when you do need accurate colours (e.g. photo editing software). I've found that it really does help to relieve eye-strain when using the PC in the evening (though sadly, to little effect on my sleep - though, of course, YMMV!) It's available for most platforms, including many smart-phones.
Of course, I encourage people to make a donation to the programmers if you find them useful and have the money - these are both very well thought out bits of software that will have taken a lot of work to produce, and it's very generous that they're available for nothing.