I tried reloading the page multiple times, the Administration page is also broken in a similar way, as well as multiple other pages. The login screen for example is not affected and looks okay.
I also tried reinstalling Bamboo altogether, even on several different servers for the course of the last couple of weeks.
This was also an issue with Bamboo 2.4.2, and possibly 2.4 ... after an upgrade some months ago it became broken in Chrome/FF.
I also tried to clean cookies, and use the Chrome incognito mode - to no avail.
I have no web filtering software on my PC that I know of, and I also tried to access the server from other people laptops and desktops - all looking the same (broken) on all.
If you require mode debugging, please tell me and I can trace it down - since its not only reproducible, but it actually just wont go away.
Problem replication
I was able to replicate the problem.
While executed from unexploded WAR, Bamboo cannot read properly mime-types file and then serves CSS files as "Content-Type: application/octet-stream"
BTW. I gave you the wrong path to the CSS to validate.
Solution
The temporary solution is to use unpackWAR="true" for Bamboo context. The final solution is of course fixing the bug in Bamboo which is my current assignment.