Implement optional customWallpaper param via JS (issue #103)

[Darthagnon]

(do not merge just yet, I want to try and get adaptive wallpapers working)

[Darthagnon]

I've been trying versions of

	<script type="text/javascript">
	function wallpaper() {
		document.body.style.backgroundImage = "url('{{ if .Site.Params.customWallpaperDark }}{{ .Site.Params.customWallpaperDark | absURL }}{{ else }}{{ .Site.Params.customWallpaper | absURL }}{{ end }}')";
		document.body.style.backgroundAttachment = "fixed";
		document.body.style.backgroundPosition = "center";
		document.body.style.minHeight = "100%";
		document.body.style.backgroundRepeat = "no-repeat";
	}

		wallpaper();
	</script>

but it's late and I'm falling asleep, without any luck.

Is there a reliable way to check whether dark mode is enabled currently? (the above doesn't, and overrides light wallpaper with dark wallpaper if present) I noticed there's a night cookie being set, but either it doesn't update very fast or I wasn't checking it right.

I'm also comparing with the Dark Mode toggle (with 2 wallpapers) from my site theme, which uses Local Storage for a live-updating var to indicate light/dark (but then, I don't know whether changing the cookie method here for that would make any difference, and there are probably dependent functions that I don't know about and would need to update)