Layers Opera Extension
Layers generates a topographic view of any web page and visualizes how its different elements are layered on top of each other.
This extension installs a small button in Opera, which when clicked, applies a custom stylesheet on the web page you are viewing. The stylesheet is constructed in such a way that you'll immediately get a better understanding of how the page's different elements relate to each other, where they overlap, and how deeply they are nested - the darker the color, the deeper the nesting. This is useful for CSS debugging, as you can see at a glance which page elements need further fine-tuning.
The latest version of Layers can be found on https://addons.opera.com/en/addons/extensions/details/layers/