ide: theme: set Post Window Text background to transparent

To match the pre-theme-management appearance, this patch sets the Post Window
Text background default value to transparent.
ide: theme: do not set background when the color is transparent

Since the transparent color can't be stored, this patch makes sure that when the
picked brush is Qt::NoBrush (~ color transparent), the background is not set.

It completes the following commit:
  ide: theme: fix transparent color