Colorize the score meter based on the score

Based on the score a linear color is computed through D3 to fill the
foreground color. I was able to reuse the code from the accessibility
diagram. In addition, I have cleared an old stylesheet where I needed
to migrate some of its rules into a general stylesheet.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Animate color for score meter

Now the color of the score meter is animated as well. This way the meter
is filled up while the color changes, too. This commit closes #44.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>