Readable gradients

Fixed

• The thank-you gradient works where browsers block reading colours from a canvas, which had turned it grey.Design
• Each theme has its own gradient, checked so text stays at 4.5:1 or better on every colour in it.Accessibility
• A still gradient in the same colours shows where WebGL is off.Design
• The gradient no longer goes blank after the card is drawn a second time.Design
• A match shows GitHub's contribution greens and a miss its danger reds.Design