Css enhancements

[andrewgoz]

This is a refactoring - the new CSS is functionally identical to the old. The primary reason for the changes is to reduce the size of the minimised CSS by nearly 20%, from 9038 bytes to 7370 bytes.

The other reason is to greatly simplify customizing series colors. Currently, the CSS styles to alter one series is:

.myclass .ct-series-a .ct-point, .myclass .ct-series-a .ct-line, .myclass .ct-series-a .ct-bar, .myclass .ct-series-a .ct-slice-donut {
  stroke: #123456;
}
.myclass .ct-series-a .ct-slice-pie, .myclass .ct-series-a .ct-area {
  fill: #123456;
}

Each additional custom series color duplicates the above over and over, adjusting for the series name.

With the new CSS variables, the required styles are:

.myclass .ct-series {
  --ct-series-a-color: #123456;
}

Each additional custom series color just duplicates the one color line, adjusting for the series name.

I have also added the "ct-chart" class name to all charts, to make it easier to write CSS rules that target all charts, in the same way there is a general "ct-series" class name.