support dark mode / dark theme

[klandergren]

The docusaurus config appears to indicate it is intentionally disabled pending design resources:

connectrpc.com/docusaurus.config.js

Lines 136 to 139 in d35ff3c

	colorMode: {
	// We can re-enable later if/when we have design assets
	disableSwitch: true,
	},

Please consider this a +1 to having a dark theme! Thank you for connectrpc and connect-web.