IMO if a website supports both light & dark themes, it should _by default_ switch automatically. If the site wants to allow overriding that, that's fine, but I hate having to constantly go into the settings of sites to switch it to "Use System Default" instead of "Light theme". Especially when sites don't store the data very long, or it's a site like AzureDevOps/Jira/etc which is deployed across multiple instances and therefore has a setting per-instance.

Depending on the device I'm using and where I am, I either let my computer automatically switch based on time of day, or keep it on dark theme. Sites that have manual toggles that don't default to respecting system settings make automatic system-wide dark/light mode an absolute pain.