5-degree theme steps, for smooth color selection

also replaces the fixed switch on theme hue with heuristic for getting
resource theme ID from the hue

the three-digit number (%03d) in the resource theme id name is needed because
the build infrastructure orders the IDs by lexical order (0,10,15,100,150,200)
instead of numerical order (0,5,10,15). By zero-padding both orders are the same
(000,005,010,015)