Add support for ivy-posframe

[ianyepan]

This pull request is needed to distinguish the different backgrounds between the posframe and the open buffer when solaire-mode is enabled.

Here's a quick before-and-after, using the scratch buffer as an example for solaire-mode. Note the lack of contrast if ivy-posframe share the same background color as the open buffer.

Before:

After:

[nashamri]

Great, thanks @ianpan870102