Looks strange when background color of mode-line-buffer-id has been defined

[kuanyui]

When :background color of mode-line-buffer-id has been defined by a theme, powerline will look strange:

The flexibility to customize the appearance of powerline would be limited by the current theme (or more accurate, the faces of mode-line, mode-line-inactive, and mode-line-buffer-id).

Maybe defface three new faces and :inherit from them to avoid this problem?

[tanzoniteblack]

Reference https://github.com/kuanyui/moe-theme.el/pull/22 instead, for powerline additions