[Accessible Slick] Prev/Next default styles don't produce visible buttons

Problem/Motivation

I've noticed that with both the standard and accessible Slick libraries, when I create a slider using the default styles and the "Prev/Next" navigation enabled, the buttons appear in the markup, but the styles from the module's slick.theme.css file don't make them visible.

Steps to reproduce

Create a new slider. I used the "Default" skin. Make sure the "Arrows" option is enabled.

Proposed resolution

The attached patch makes some minor changes to the arrow styles that should produce visible buttons. The unicode characters used do not rely on the Slick font, and I believe should work in most fonts.

Remaining tasks

I'm not certain whether this change will be disruptive to existing installations. I'm hoping people will test it and offer feedback.