border-dashed Doesn't Work

[neildaniels]

Describe the bug
The border-dashed doesn't work correctly.

https://primer.style/css/utilities/borders#Border-style

Screenshots

Desktop (please complete the following information):

• OS: macOS 10.14.5 (18F132)
• Safari 12.1.1 (14607.2.6.1.1)

[simurai]

@neildaniels Thanks 🙇

I think the problem is that .border-dashed gets overridden by the responsive variant mixin. We probably can move .border-dashed below and it should work again.