Theme detection

[Nemo157]

First step of #918

Moves all the colours out of the style files and uses CSS variables instead, applying a default set on html, then adds a couple of snippets that detect the current rustdoc theme and copies it to a data-theme variable on html, allowing overriding the CSS variables to change the theming.

Next step after this is to add overrides for all the pure specified colours, using CSS variables, so that we can change them; then adding colour sets matching the current dark and ayu rustdoc themes.

[Nemo157]

cc @Cldfire if you have any thoughts

[Cldfire]

From my perspective it looks great! The simple variable definitions will make it really easy to add themes. Thank you :)

[jyn514]

Only other windows get notified when we change local storage

Who came up with that idea??? 🤦

Well, if that's the case this looks fine.

[Nemo157]

Yeah, and I was very surprised to find out that neither dedicated or shared workers get the events either, even though they should be treated as a separate context, requiring going all the way to loading another document in an iframe.

[jyn514]

Should probably be

Suggested change

	--color-border-light: lighten(#ddd, 5%);
	--color-border-light: lighten(var(--color-border), 5%);

although not sure if that syntax works.

[Nemo157]

Nope, lighten is an SCSS function which needs the value at compile time, but lucky you mentioned it cause it turns out that function doesn't run in this context for some reason and instead lighten is put into the output CSS, I think we need to just precalculate the values here.

[jyn514]

Thanks for working on this :)