Hey @skebanga
This is something you need to override with some custom CSS. I answered a similar question here, and have been working on some general stylesheets for bootstrap and bootswatch that will style dash-core-components consistently, but I haven’t had time to finish them. If I remember I’ll comment on this thread again when I’ve made some progress with that. In the meantime the linked post above will hopefully help a bit.
EDIT - I’m not 100% sure if the code linked above will work perfectly with the latest version of dash-core-components. I know that the styling of the DatePicker components changed, I can’t remember if the same happened for Dropdown which might mean you need to tweak the CSS classes.