The actual panel with options is not part of the website and the website only has very limited control of styling of the items inside it. You can really only change the background-color of the panel, or individual options as well as the foreground-color.

The panel is part of the Firefox window and as such you would use browser toolbox with option to disable popup autohide after which you can inspect it via browser toolbox. But it is inaccessible to website devtools.