picotui should not assume a particular terminal "theme"

[pfalcon]

As the video in #50 shows, picotui doesn't properly set all character attributes, and at places, they look weird (white-on-black). That's of course because I use Gnome's default black-on-white theme, and for me it works as it should. Again, should not make such assumptions, and set all attributes explicitly.

(Maybe just set black-on-white as the default attribute, if that's possible, but I'm not sure white-on-black users will be happy (if it persists after quit)).