CSS: Consistent styling between Bulma and dcc components

[mkhorton]

It would be useful to be able to use a combined form control of a Select (dropdown) + a button, see:

https://bulma.io/documentation/form/general/#form-addons

However we're using the dcc.Dropdown component for this (since this is React-ready and has Dash integration).

It would be useful to be able to re-style this component so that it's consistent with the Bulma style, if possible.

The intent for the combined form control is to be able to select options e.g. filetype associated with a dropdown button; we could also try an alternative to this if something else makes sense.