Update icon set color to `$color-mid-dark`

[kwm14]

Describe the bug Check that all icons in Vanilla use the correct value #666 or $color-mid-dark

To Reproduce Steps to reproduce the behavior:

1. Go to https://docs.vanillaframework.io/en/
2. Click on https://docs.vanillaframework.io/en/patterns/icons
3. Inspect the color of each icon so it matches value above

[kwm14]

Investigation

From inspecting our icon set and revisiting the SVG's themselves I discovered 12 of the icons we're using 333333 and 808080 hex values.

• p-icon--user
• p-icon--share
• p-icon--search
• p-icon--plus
• p-icon--minus
• p-icon--menu
• p-icon--external-link
• p-icon--expand
• p-icon--delete
• p-icon--code
• p-icon--close
• p-icon--chevron

I have updated in a new Sketch file and will update icons in the framework 👍