Control the background colour and font of the index and header

[DiegoF90]

We are currently able to modify the background colour of individual cells. Please would it be possible to have an option to control the table's header and index colour as well as the font?

This would be helpful as a workaround for #249

[DiegoF90]

For anyone else looking for a workaround for #249, I ended up modifying the css manually with:

display(HTML(r"<style>.handsontable th {color: white; background-color: gray;}</style>"))

where display and HTML are both in from IPython.display