Make private tab color predominant

[cezaraugusto]

Auditors: @bsclifton

Fix #7720
Fix #7715

/cc @bradleyrichter

Test Plan:

• If a tab is private, it should be purple even if themeColor is defined

[bsclifton]

Changes look great 😄 ++

[bsclifton]

I was playing around with session tabs for comparison and noticed a bug (similar in nature to this)

If you open a new session tab and switch to it, the close button is white when you do a mouse over:

[jonathansampson]

@bsclifton I'm not able to repro the New Session Tab issue you're seeing:

[jonathansampson]

I see no issues in my quick test, but @bsclifton did.

[cezaraugusto]

@bsclifton can't repro. Can you give a more detailed STR?

[bsclifton]

weird- definitely can't repro now...

Before I tested (both today and yesterday), I did rebase against master. Yesterday may have had a conflict causing the white text; it works great now 😄

[bsclifton]

@cezaraugusto if you can go ahead and rebase, I'll merge (assuming @jonathansampson didn't have any objections 😄 )

[cezaraugusto]

thanks rebased