color-scheme

[talltotal]

<meta name="color-scheme" content="light dark">
<meta name="supported-color-schemes" content="light dark">

[willpower232]

I had no idea this existed seems very neat based on https://web.dev/color-scheme/

Apparently supported-color-schemes was the original way of doing it but isn't in the spec any more (https://bugzilla.mozilla.org/show_bug.cgi?id=1525107#c1)