Improve dark mode

[tbl0605]

Hi, first of all thank you for this great library! :)

I have a small suggestion for improving dark mode support. Some CSS themes I've tested (in my case, some commercial boostrap themes) reset the default background color of HTML buttons (even if it's just to set it to white).

Adding following CSS code to the base.css file would preserve the background color of the calendar and date picker header buttons:

.vc-header button {
  -webkit-appearance: button;
  background-color: transparent;
  background-image: none;
}