Dark theme fixes, plus better indication of which page is active

Fixes the appearance of the scanner options and document pages in a dark colorscheme. Previously they did not properly follow system colorscheme.

Also changed border color of active page to Kirigami.Theme.focusColor, to make it clear which one is active.

Also made the scanner options form labels not-bold, making them consistent with forms in other apps. Also made scanner options horizontally centered, again making them consistent with other KDE apps.

Screenshots:

Scanner options (before):

Scanner options (after):

Page (before):

Page (after):