The site was very dark even though my system is set to light mode.
Maybe it would be better to set the settings to something like this:
so that both dark and light users can be happy? The user’s system preference should be detected. 
Indeed, the first thing I did was switch my theme.
There should now be a default theme that switches between light and dark mode depending on your system settings if your device supports it – it seems to work on my phone but not my desktop Firefox. Let me know if that seems like a good solution. There are a lot of buttons on the Admin Panel to play around with.
If you want to change your own per-user settings, you can find the page that @BSchilperoort referenced under the “Interface” tab on your user preferences (https://forum.csdms.io/u/$USER_NAME/preferences).
I can’t check if it’s fixed, because I changed my own user settings and I don’t see a way to reset those to default. But it should be good now.
Glad it’s not just me who changed the theme to light straight away 