[FIX] Interface: Enhance readability of grey text

RT­2243 #Deliver
In multiple places of the interface, #ccccca @midgray is used as text
color. This is hard to read, especially on lightgray backgrounds.
Replace color: @midgray with @darkgray where suitable, enhance navbar
and subnav hover styles.
Merge pull request #1570 from jublonet/feature/2243-bg-color-saturation

[FIX] Interface: Enhance readability of grey text