Seperate tooltip background property for custom styles.

Hi,

I have set a custom style for my charts which looks great but with transparency it unfortunately makes the tooltip difficult to read as it also becomes transparent. I cannot see an exisiting property for this so (unless I have missed it - appolgies if I have) this is a feature request.

My custom style is largley the same as the one in the [docs](http://www.pygal.org/en/stable/documentation/custom_styles.html#using-style-class) and this issue with the tooltip can also been seen on the chart given as an example on the page.

I may attempt to have a look at adding this myself but I am new to Python which I am learning for this project. Hopefully it is not too difficult to add.

Thanks

Start with the custom styles example in the linked documentation and compare the tooltip's appearance when the chart style uses transparency. Identify how tooltip styling is configured, then make its background independently configurable and verify that the tooltip remains readable without changing the chart's other transparent styles.

Written by the indexing model from the issue text.