Add support for themes

[iragm]

Is your feature request related to a problem? Please describe.
This is a nice tool and easy to use, however it doesn't seem to work with bootstrap themes. For example, with a dark theme, the picker's background is white and the theme I'm using makes the text white, so nothing shows up.

Describe the solution you'd like
I can add to my app's CSS something like this:

.bootstrap-datetimepicker-widget {
    background-color: #222 !important;
}

This works for the project I'm working on now, but this is a bit clumsy and it would be nice for themes to be supported without needing to write custom CSS.

Describe alternatives you've considered
See above.

Additional context
Themes are a pretty important part of bootstrap and I'm surprised they're not better supported. Maybe they are and I'm just overlooking it?

This is the theme I'm using: https://bootswatch.com/darkly/