JENKINS-20551 - Display improvements for claimed builds and progress views

This fixes JENKINS-20551 by replacing the claim border with a background hatching, so that the text is always in the same position in all jobs. It also re-introduces gradient styling which appears to have been broken by a recent push, but with a slightly cleaner gradient to avoid overlapping the job text with a change in gradient. Progress bars have also been updated to improve definition from non-progress bars and allow slightly more vibrant colours for jobs that aren't currently building (which usually means jobs requiring attention).
The code for themeing has been simplified to use CSS for everything other than animations. 

Example output image attached to: https://issues.jenkins-ci.org/browse/JENKINS-20551