Add contrast function

[cibersheep]

• Use contrast to decide text color on button for Ambiance
• Darker color if clear background

fixes: #59

[bhdouglass]

This looks good to me (other than the build seems to be failing, for testing I did before and after screenshots with various buttons:

Before:

After:

[cibersheep]

Thanks Brian.

• I see that some colors as Ash or Red are in the limit of the contrast ratio but most of them benefit of the new behaviour.
• I have MRed some changes for the theme palette to address the red, blue and green (unrelated to this MR though)

[cibersheep]

I'm fine with contrastRatio() by itself, I just wanted the other name to be changed. Anyway, as long as the other method is not public, I'm fine :-)

Oh, I see. I'm sorry I must misread the whole message.