Fix web browser's close tab buttons in Windows 8 camouflage

The tab close button, the “x”, is red, greatly contrasting to the blue. In the Internet Explorer 10 I looked at (which OTOH has a very light gray tabs) the x is dark gray, which I think would look better. If that proves hard to fix, removing them by setting browser.tabs.closeButtons = 2 is otherwise ok.

Feature Branch: feature/7367-browser-camouflage

Parent Task: #6342 (closed)

Original created by @anonym on 7401 (Redmine)