This is another feature that's been requested a number of times, and was implemented as an open-source contribution by @Celeo.
If you're not aware (since apparently a fair number of people haven't noticed them), there are currently 4 different color themes for the site that you can choose from on the settings page. Until now, this choice has always only applied to the specific device that you're on, and is stored in a cookie, so people that switch devices often or clear their cookies were a bit annoyed by needing to re-select their theme every time they log in.
Now you can set a default theme for your account, so when you log into your account it will automatically change to that theme (but you can still override it if you want to use a different theme on that device). To do this, just change to the theme that you want to use as your default on the settings page, and a button to set as default should show up next to the dropdown. The behavior of it's still a little bit weird in some cases, so you may need to change the dropdown away and back if it's already set to the one you want to use.