I’ve been experimenting with extending Bootstrap CSS to add consistent styling to Dash core components. You can check that out here. Like I say, it’s somewhat experimental, and there’s a couple of bootswatch themes that the support isn’t perfect for, but I think it’s working well enough at the moment that it will get you at least 90% of the way there if you’re using one of the dark Bootswatch themes for example and you want a dark Dropdown to match.
If you do try it out, I’m very happy for people to raise issues / pull requests where they encounter issues, so do let me know how you get on.