Improve default style & settings (e.g. removing chosen value)

Problem/Motivation

Trying the module with default settings leads to frustration, as for example it's hard to understand, how to remove selected options. The "x" is missing, as I think it's disabled in the settings.

Also, the styling might be improved to better match Drupal Core themes CSS.
For a demo what to expect, for example replacing Drupal Views multiple select filters, see https://choices-js.github.io/Choices/

To get a feeling, what I mean:
After enabling the module and using it for a views exposed filter, it looks like this, with both node types selected:

I didn't know how to unselect them and would have expected to have the input behind the selected options e.g.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes