Get accent from settings portal

[danirabbit]

What Happened?

Currently we get the accent color from the stylesheet, which means if the stylesheet gets changed from one of the elementary ones, our accent color stuff breaks

Steps to Reproduce

1. Change the stylesheet to something like Adwaita
2. See that highlighting in the alt tab window switcher is broken

Expected Behavior

There's now an accent color on the settings portal, so that seems the cleaner way to get the accent color. See: elementary/settings-daemon#60

OS Version

7.x (Horus)

Software Version

Latest release (I have run all updates)

Log Output

No response

Hardware Info

No response