Ayo. A victory too quickly declared, see Updates.
More testing with Chrome on Mac and Windows, as well as Firefox on Windows showed issues with the theme switching. I now don’t see one stylesheet (theme) quickly showing up, to be replaced by the actually selected one, but the contents with no stylesheet applied for a split-second, before the selected stylesheet is applied, of course causing an unpleasant flickering.
So even though the correct <link> to the stylesheet is in the <head> section, which can easily verified with the browser’s inspector, some browsers start to render (“paint”) the contents before the stylesheet is ready (parsed etc.). Maybe modern browsers are too smart, parallelising as much as possible, for this kind of approach. It seems to be a timing issue.
I’ll have to investigate. That’s what you – in this case, I – get for trying to implement fancy features. And skimping on thorough testing. Oh well.
For now, theme switching is disabled with this build of the site.
Enjoy the new shiny dark theme in the meantime.