The default infobox is great for a a quick and dirty out-of-the-box experience, but we wound up disabling it (
infoBox: false option to the Viewer constructor) and making our own by hooking
Viewer#selectedEntityChanged. If you’re otherwise happy with how the infobox works, you can try to fix this, but if you think you may wind up needing further customizations you might want to create your own panel now and save the headache.
One of the main reasons that we rolled our own info viewer was that the built-in one has sandboxing to allow safe viewing of untrusted content. For example, if you get your entity info from a WMS feature service or a public KML blob, the blurb might be HTML with script tags, which would open you up to an XSS attack if you just inject it directly into the page. That’s why Cesium uses a protected iframe. Unfortunately, that also makes it really hard to apply consistent styling, in my experience. If you’re creating all the info content yourself, you don’t need the iframe, which makes styling a heck of a lot simpler.