Adaptions after Xfce 4.20 release and additional small changes

The release of Xfce 4.20 caused two issues: Theme and wallpaper. To stay at theme "Greybird", it needs to be set explicitely. For the wallpaper I unfortunately haven't found a possibility to set the one of postmarketOS as default.

While at it, I added additional small changes to the layout, some of them to take into account the increasing number of hight-resolution touchscreen devices.