Fluxer websites should respect system themes by default

Currently, both the fluxer.app and fluxer.com websites use a dark theme by default. That is unless the user has cookies enabled and sets the site theme manually.

I understand this probably makes 90% of visitors happy and might come off as nitpicky. However, I do think it'd be better fo everyone if the sites defaulted to the "system" theme instead of enforcing one or the other until the user sets a theme themselves. That also allows visitors to have cookies disabled while the content adapts to their preference or needs. The website should respect the system theme by default, instead of defaulting to dark mode.