You should make use of template images wherever possible because they allow your UI to automatically adapt to changes made by the system (at least when there's not a bug in the system... http://indiestack.com/2014/10/yosemites-dark-mode/). But in the case where you might use a custom view in the status bar and cannot take advantage of a template image, you can manually check for dark mode and adapt your UI accordingly.
You can check whether or not dark mode is enabled by retrieving a key from the user's global preferences, like this:
NSDictionary *dict = [[NSUserDefaults standardUserDefaults] persistentDomainForName:NSGlobalDomain];
id style = [dict objectForKey:@"AppleInterfaceStyle"];
BOOL darkModeOn = ( style && [style isKindOfClass:[NSString class]] && NSOrderedSame == [style caseInsensitiveCompare:@"dark"] );
At least for the first release of Yosemite, the key is not present when dark mode is disabled, but the key is present and returns the string value @"Dark" when dark mode is enabled. I added the case insensitive compare because I have seen preference keys change their case between system releases, and this adds a little insurance against that.
To monitor the current state of the setting, you register as an observer of a distributed notification (within an appropriate method), like this:
[[NSDistributedNotificationCenter defaultCenter] addObserver:self selector:@selector(darkModeChanged:) name:@"AppleInterfaceThemeChangedNotification" object:nil];
And you create a method to act as the message selector for the notification, like this:
-(void)darkModeChanged:(NSNotification *)notif
{
NSLog(@"Dark mode changed");
}