I noticed there was a setting called "Website appearance" which doesn't have an automatic day/night cycle for switching, like based on time or sunrise/sunset.
Also, I would love to have the browser to adopt whatever system accent is being used. In my case KDE. Currently the only way for me to even change colours on the browser is a full blown theme. How about some advanced option like KDE theme editing?
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Regarding the accent colour, see here too: