* Fix contrast calculation for thumbnail color extraction
Luminance calculation was using 0-255 RGB values instead of 0-1 sRGB values,
leading to incorrectly-computed contrast values.
Since we use ColorDiff already, just use its XYZ colorspace conversion code
to get the value.
* Require at least 3:1 contrast for both accent and foreground colors
* Lower required contrast for the accent color
0ebc8d8626