707 Add theming file

[jaygiang]

No description provided.

[thgaskell]

I think the useRadFishConfig functionality could be pulled into vite-plugin-radfish-theme.js to expose constants directly from Vite.

So inside the template code it could reference values, rather than relying on the config provider:
Example:

<h1>{import.meta.env.RADFISH_APP_NAME}<h1>

Additionally, I think the plugin could also make use of the closeBundle hook to write RADFish theme values directly to the manifest.