Default Theme Should Respect System Accent Color

[vincentj]

(Copying this over from the Ideas site.)

Windows provides users with the ability to customize an accent color for window titlebars.
Firefox should support this accent color for the tab strip.

On Windows 10, the option is enabled under:
Settings > Personalization > Colors > Show accent color on the following surfaces > Title bars and window borders

The color for inactive windows can also be adjusted via the registry, and Firefox should also use that color when it has a value to support better accessibility.
(HKCU\SOFTWARE\Microsoft\Windows\DWM\AccentColorInactive)

[AshenPaladin]

Thunderbird seems to now use and respect your Windows accent color - it's only to a certain degree, true, but it's much more so than what Firefox is doing.