The custom module and the client app can only communicate with osc messages, there’s no such thing as shared variables or pointers between the two of them. You could use the osc listener syntax as explained here to send a value from the custom module to the client app.
Note that when it comes to css, writing a theme file is usually the best way to avoid code repetition; it can also be very efficient combined with widget extra css classes. You could keep all your svg in the css file or even as svg files alongside the css file.