This is the patch.
Note that to NOT change the static methods brighten and darken I have put the new parameter (colorMultiplier) as a static variable inside the class.
Tell me what do you think on this, and in any case you are free to apply instead of me and change (the variable name "colorMultiplier" probably is not the best).
If no objections, I'll commit the patch (but not before the end of this week).
To see that it works and what happens, try to manually changing the default value for example to 0.2 gives a more contrasted theme.
After this, I'll set the default value even in all other Pivot json themes file (currently there isn't a way to not specify it and catch the exception to leave the default value set in code).
Verify after this (for 2.1) if it's the case to check the given value in the range 0.0, 1.0 (normalized) and give an exception in case of a given wrong value.