JetBrains: Update web view theme when the theme changes

Created by: vdavid

Context

We currently check the theme at the web view start and apply some CSS to make it fit in.

Problem

The user might change the theme later. If the web view doesn't adjust accordingly, it'll look out of place.

Solution

Listen to theme change events. Create a way to elegantly call the JS from a Java. (https://github.com/sourcegraph/sourcegraph/issues/34797 should help) Call JS when the theme changes.