tasks #7127 Use theme color selected in Tuleap (1st step)

This is a tasks from story #6003: be able to change the Experimental theme color

* Use a body class in order to use color
* Waiting for tasks #7123 in order to use database preferences*

To use it, add 'blue' class in body (color not definitive, they are here for test purpose)

Change-Id: Ia3e33bbc3464fcacc5aade7c1ea96903028e3c10
Merge commit 'refs/changes/93/2693/4' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #7127 Use theme color selected in Tuleap (1st step)
This is Tuleap 7.2.99.4

Change-Id: I1502d3173035ec9c442f2f14d395e6d97ace5334