ui: rename "Theme" selector to "Color Theme"

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>

www/manager6/Workspace.js

@@ -397,7 +397,7 @@ Ext.define('PVE.StdWorkspace', {
 				},
 				{
 				    iconCls: 'fa fa-paint-brush',
-				    text: gettext('Theme'),
+				    text: gettext('Color Theme'),
 				    handler: function() {
 					Ext.create('Proxmox.window.ThemeEditWindow')
 					    .show();