Customise | Customise User Interface | Colours (tab)
Expand the Drop-down list which says "Viewports" and select "Trackbar".
In the listbox below that, select "Trackbar Background".
Max 2016 (SP1/EXT1)
Win7Pro x64 (SP1). i5-3570K @ 4.4GHz, 8Gb Ram, DX11.
nVidia GTX760 (2GB) (Driver 430.86).