rename "Theme" selector to "Color Scheme"

Having "Color" added makes it easier to translate (i.e. Farbschema,
配色) and at least as understandable as Theme, so change it,

Suggested-by: Markus Frank <m.frank@proxmox.com>
 [ T: while Markus suggested Color Scheme, the hive-mind opted for
 this ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>