This uses -XOverlappingInstances to provide a fallback instance which uses the
focusedBorderColor and normalBorderColor, but that part is optional.
User's configs should use -XNoMonomorphismRestriction if they want to avoid
writing a type signature for myGSConfig.
Also, type variables become ambiguous in expressions like:
> myGSConfig = defaultGSConfig { gs_navigate = neiu `M.union` gs_navigate defaultGSConfig }
> where neiu = M.map (\(x,y) (a,b) -> (x+a,y+b)) $ M.fromList
> [((0,xK_n),(-1,0)) ,((0,xK_e),(0,1)) ,((0,xK_i),(1,0)) ,((0,xK_u),(0,-1))]
But that can be resolved with the appropriate (`asTypeOf`myGSConfig) applied to
the second defaultGSConfig, or the use of some other method for modifying
existing fields.