Caret is a rectangle with Windows High Contrast Mode

Created by: Zauberbutter

Information

• Operating system and version: Windows 10 Home
• Browser and version: Firefox 63.0.1 & IE 11.1.17134.0

Bug

When using transparent borders they aren't transparent with Windows High Contrast Mode, so you can't see that caret is a caret:

Steps to reproduce

1. Open Firefox or IE
2. Navigate to http://getbootstrap.com/docs/4.0/components/dropdowns/
3. Enable Windows High Contrast Mode (left) shift + (left) alt + print