Do not override themeColor if platform is Windows

[cezaraugusto]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Auditors: @bbondy

Fix #7216

The main issue was that Windows specific style was being declared after themeColor and late declared classes take precedence so themeColor was being overridden on Windows.

Test plan:

• Test it on Windows
• Enable themeColor for tabs on settings
• Go to https://facebook.com
• Title color should be visible based on tab's background (getTextColorForBackground()) and not #555 (Windows specific color)

[bsclifton]

Confirmed this works great on Windows 😄 ++