Theme name and id mismatch is not handled intuitively/consistently

steps to reproduce:
1. create a theme named "demo-theme"
2. change the theme id from "demo" to "my-demo" in liferay-look-and-feel.xml (it just has to be something different to "demo"
3. deploy the theme
Result:

• Log says (multiple times) "No theme found for specified theme id my-demo. Returning the default theme."
• CSS variable resolution of (e.g.) @theme_image_path@ will return image path for classic theme (as this is what the message expressed).

Granted, steps 1 and 2 are an "artificial" way to reproduce, but this will also happen upon renaming the theme without changing the id.

Proposed solution is to either exclusively use the id or the theme context name - any will suffice. Another proposed solution is to change the theme build process to fail or generate a well-visible warning that the theme will not work when name/id don't match.