Dynamic theming. Closes #241.

Upgrades to Roact master and introduces dynamic theme switching.

We branch on the theme name in order to try to use Rojo's brand
colors instead of Studio's. I kind of winged it with these colors
and we might want to choose slightly nicer dark theme colors
in the future.

I also took the opportunity to reorganize the color naming scheme
since it didn't really work for dark theme.

plugin/src/init.server.lua

@@ -19,7 +19,7 @@ local app = Roact.createElement(App, {
 	plugin = plugin,
 })
 
-local tree = Roact.mount(app, game:GetService("CoreGui"), "Rojo UI")
+local tree = Roact.mount(app, nil, "Rojo UI")
 
 plugin.Unloading:Connect(function()
 	Roact.unmount(tree)