Double border on select elements with slim-select enabled

[msnassar]

I have noticed that the double border issue that has been reported here https://citnet.tech.ec.europa.eu/CITnet/jira/browse/OEL-2255, has been fixed in oe_whitelabel here https://github.com/openeuropa/oe_whitelabel/pull/236/files instead of fixing it in BCL (openeuropa/bootstrap-component-library#515 (comment))

After applying the solution to UCPKN, I see 2 issues:

1. We lose the border on the corners. See the screenshot
   
   The fix we did in ucpkn:

.ss-main.multi-select.form-select {
  border: none;
}

2. (Trivial) Since the fix is not part of the BCL, we still have to explicitly fix the issue in ucpkn_theme. This is because the ucpkn styles comes after oe_whitelabel styles. I don't know whether the other sites will encounter the same issue or not.