Update UI spec for private tabs

[cezaraugusto]

Test plan

1. Open a new private tab
2. Switch between private tab and a regular tab. When not active, the private tab should not be purple
3. When active, the private tab should always be purple
4. Private tab should always have the 🕶 icon

Original issue description

Per @bradleyrichter request:

1. Private tabs should have its color (purple) only when active, no hoverState
2. Private tabs color should override themeColor
3. Replace 👁 icon with 🕶

[bbondy]

No new UI changes pls once we're in RC for that milestone. Unless it's a regression or big problem. I think it's neither of those so I updated to 0.14.1