Just a PSA that since 10.2.2 -> 10.3.1 the Vizportal.js and Vizportal.min.js files seem to have been rewritten.
We've used these to customize the header colours from the default 'Venice Blue' to a nice black.
Below are the current methods I've used to change our headers again in 10.3.1.
Also note, if like me you've copied and pasted the edited JS file while upgrading your Tableau Server. This time around it will result in the pages loading completely blank. Hitting F12 (Developer mode displays a couple errors which mention the Vizportal file.
The instructions below are for black, you can set the RGB to anything.
The header background colour is controlled in two places, vizportal.min.js and vizportal.js
Both vizportal and vizportal.min have .gz files which are zipped backups.
Original Credit goes to Nick Pileggi (I believe)